Utilize per-vma locks to stabilize vma after lookup without taking
mmap_lock during PROCMAP_QUERY ioctl execution. If vma lock is
contended, we fall back to mmap_lock but take it only momentarily
to lock the vma and release the mmap_lock. In a very unlikely case
of vm_refcnt overflow, this fall back path will fail and ioctl is
done under mmap_lock protection.

This change is designed to reduce mmap_lock contention and prevent
PROCMAP_QUERY ioctl calls from blocking address space updates.

Signed-off-by: Suren Baghdasaryan <surenb@google.com>
---
 fs/proc/task_mmu.c | 84 +++++++++++++++++++++++++++++++++++++---------
 1 file changed, 68 insertions(+), 16 deletions(-)

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 45134335e086..0396315dfaee 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -517,28 +517,81 @@ static int pid_maps_open(struct inode *inode, struct file *file)
 		PROCMAP_QUERY_VMA_FLAGS				\
 )
 
-static int query_vma_setup(struct mm_struct *mm)
+#ifdef CONFIG_PER_VMA_LOCK
+
+static int query_vma_setup(struct proc_maps_locking_ctx *lock_ctx)
 {
-	return mmap_read_lock_killable(mm);
+	lock_ctx->locked_vma = NULL;
+	lock_ctx->mmap_locked = false;
+
+	return 0;
 }
 
-static void query_vma_teardown(struct mm_struct *mm, struct vm_area_struct *vma)
+static void query_vma_teardown(struct proc_maps_locking_ctx *lock_ctx)
 {
-	mmap_read_unlock(mm);
+	if (lock_ctx->mmap_locked)
+		mmap_read_unlock(lock_ctx->mm);
+	else
+		unlock_vma(lock_ctx);
+}
+
+static struct vm_area_struct *query_vma_find_by_addr(struct proc_maps_locking_ctx *lock_ctx,
+						     unsigned long addr)
+{
+	struct vm_area_struct *vma;
+	struct vma_iterator vmi;
+
+	if (lock_ctx->mmap_locked)
+		return find_vma(lock_ctx->mm, addr);
+
+	unlock_vma(lock_ctx);
+	rcu_read_lock();
+	vma_iter_init(&vmi, lock_ctx->mm, addr);
+	vma = lock_next_vma(lock_ctx->mm, &vmi, addr);
+	rcu_read_unlock();
+
+	if (!IS_ERR_OR_NULL(vma)) {
+		lock_ctx->locked_vma = vma;
+	} else if (PTR_ERR(vma) == -EAGAIN) {
+		/* Fallback to mmap_lock on vma->vm_refcnt overflow */
+		mmap_read_lock(lock_ctx->mm);
+		vma = find_vma(lock_ctx->mm, addr);
+		lock_ctx->mmap_locked = true;
+	}
+
+	return vma;
+}
+
+#else /* CONFIG_PER_VMA_LOCK */
+
+static int query_vma_setup(struct proc_maps_locking_ctx *lock_ctx)
+{
+	return mmap_read_lock_killable(lock_ctx->mm);
 }
 
-static struct vm_area_struct *query_vma_find_by_addr(struct mm_struct *mm, unsigned long addr)
+static void query_vma_teardown(struct proc_maps_locking_ctx *lock_ctx)
 {
-	return find_vma(mm, addr);
+	mmap_read_unlock(lock_ctx->mm);
 }
 
-static struct vm_area_struct *query_matching_vma(struct mm_struct *mm,
+static struct vm_area_struct *query_vma_find_by_addr(struct proc_maps_locking_ctx *lock_ctx,
+						     unsigned long addr)
+{
+	return find_vma(lock_ctx->mm, addr);
+}
+
+#endif  /* CONFIG_PER_VMA_LOCK */
+
+static struct vm_area_struct *query_matching_vma(struct proc_maps_locking_ctx *lock_ctx,
 						 unsigned long addr, u32 flags)
 {
 	struct vm_area_struct *vma;
 
 next_vma:
-	vma = query_vma_find_by_addr(mm, addr);
+	vma = query_vma_find_by_addr(lock_ctx, addr);
+	if (IS_ERR(vma))
+		return vma;
+
 	if (!vma)
 		goto no_vma;
 
@@ -579,11 +632,11 @@ static struct vm_area_struct *query_matching_vma(struct mm_struct *mm,
 	return ERR_PTR(-ENOENT);
 }
 
-static int do_procmap_query(struct proc_maps_private *priv, void __user *uarg)
+static int do_procmap_query(struct mm_struct *mm, void __user *uarg)
 {
+	struct proc_maps_locking_ctx lock_ctx = { .mm = mm };
 	struct procmap_query karg;
 	struct vm_area_struct *vma;
-	struct mm_struct *mm;
 	const char *name = NULL;
 	char build_id_buf[BUILD_ID_SIZE_MAX], *name_buf = NULL;
 	__u64 usize;
@@ -610,17 +663,16 @@ static int do_procmap_query(struct proc_maps_private *priv, void __user *uarg)
 	if (!!karg.build_id_size != !!karg.build_id_addr)
 		return -EINVAL;
 
-	mm = priv->lock_ctx.mm;
 	if (!mm || !mmget_not_zero(mm))
 		return -ESRCH;
 
-	err = query_vma_setup(mm);
+	err = query_vma_setup(&lock_ctx);
 	if (err) {
 		mmput(mm);
 		return err;
 	}
 
-	vma = query_matching_vma(mm, karg.query_addr, karg.query_flags);
+	vma = query_matching_vma(&lock_ctx, karg.query_addr, karg.query_flags);
 	if (IS_ERR(vma)) {
 		err = PTR_ERR(vma);
 		vma = NULL;
@@ -705,7 +757,7 @@ static int do_procmap_query(struct proc_maps_private *priv, void __user *uarg)
 	}
 
 	/* unlock vma or mmap_lock, and put mm_struct before copying data to user */
-	query_vma_teardown(mm, vma);
+	query_vma_teardown(&lock_ctx);
 	mmput(mm);
 
 	if (karg.vma_name_size && copy_to_user(u64_to_user_ptr(karg.vma_name_addr),
@@ -725,7 +777,7 @@ static int do_procmap_query(struct proc_maps_private *priv, void __user *uarg)
 	return 0;
 
 out:
-	query_vma_teardown(mm, vma);
+	query_vma_teardown(&lock_ctx);
 	mmput(mm);
 	kfree(name_buf);
 	return err;
@@ -738,7 +790,7 @@ static long procfs_procmap_ioctl(struct file *file, unsigned int cmd, unsigned l
 
 	switch (cmd) {
 	case PROCMAP_QUERY:
-		return do_procmap_query(priv, (void __user *)arg);
+		return do_procmap_query(priv->lock_ctx.mm, (void __user *)arg);
 	default:
 		return -ENOIOCTLCMD;
 	}
-- 
2.50.1.565.gc32cd1483b-goog

On Wed, Aug 06, 2025 at 08:59:04AM -0700, Suren Baghdasaryan wrote:
> Utilize per-vma locks to stabilize vma after lookup without taking
> mmap_lock during PROCMAP_QUERY ioctl execution. If vma lock is
> contended, we fall back to mmap_lock but take it only momentarily
> to lock the vma and release the mmap_lock. In a very unlikely case
> of vm_refcnt overflow, this fall back path will fail and ioctl is
> done under mmap_lock protection.
>
> This change is designed to reduce mmap_lock contention and prevent
> PROCMAP_QUERY ioctl calls from blocking address space updates.
>
> Signed-off-by: Suren Baghdasaryan <surenb@google.com>

A lot of nits but nothing's really standing out as broken, AFAICT...

> ---
>  fs/proc/task_mmu.c | 84 +++++++++++++++++++++++++++++++++++++---------
>  1 file changed, 68 insertions(+), 16 deletions(-)
>
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index 45134335e086..0396315dfaee 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -517,28 +517,81 @@ static int pid_maps_open(struct inode *inode, struct file *file)
>  		PROCMAP_QUERY_VMA_FLAGS				\
>  )
>
> -static int query_vma_setup(struct mm_struct *mm)
> +#ifdef CONFIG_PER_VMA_LOCK
> +
> +static int query_vma_setup(struct proc_maps_locking_ctx *lock_ctx)
>  {
> -	return mmap_read_lock_killable(mm);
> +	lock_ctx->locked_vma = NULL;
> +	lock_ctx->mmap_locked = false;

We also do this in lock_vma_range(), seems sensible to factor out? E.g.:

static void ctx_clear_vma(struct proc_maps_locking_ctx *lock_ctx)
{
	lock_ctx->locked_vma = NULL;
	lock_ctx->mmap_locked = false;
}

> +
> +	return 0;
>  }
>
> -static void query_vma_teardown(struct mm_struct *mm, struct vm_area_struct *vma)
> +static void query_vma_teardown(struct proc_maps_locking_ctx *lock_ctx)
>  {
> -	mmap_read_unlock(mm);
> +	if (lock_ctx->mmap_locked)
> +		mmap_read_unlock(lock_ctx->mm);

Maybe worth a comment as to why we leave lock_ctx->mmap_locked set here?

> +	else
> +		unlock_vma(lock_ctx);

Should have said on 2/3, but I wonder if we should prefix with ctx_, as
'unlock_vma()' and 'lock_vma()' seem like core functions... esp. since we
have vma_start_read/write() rather than functions that reference locking.

So - ctx_unlock_vma() and ctx_lock_vma() or unlock_ctx_vma() /
lock_ctx_vma()?

> +}
> +
> +static struct vm_area_struct *query_vma_find_by_addr(struct proc_maps_locking_ctx *lock_ctx,
> +						     unsigned long addr)
> +{
> +	struct vm_area_struct *vma;
> +	struct vma_iterator vmi;
> +
> +	if (lock_ctx->mmap_locked)
> +		return find_vma(lock_ctx->mm, addr);
> +
> +	unlock_vma(lock_ctx);
> +	rcu_read_lock();
> +	vma_iter_init(&vmi, lock_ctx->mm, addr);
> +	vma = lock_next_vma(lock_ctx->mm, &vmi, addr);
> +	rcu_read_unlock();

I think a comment at the top of this block would be useful, something like
'We unlock any previously locked VMA and find the next under RCU'.

> +
> +	if (!IS_ERR_OR_NULL(vma)) {

Is the NULL bit here really necessary? presumably lock_ctx->locked_vma is
expected to be NULL already, so we're not overwriting anything here.

In fact we could get rid of the horrid if/else here with a guard clause like:

	if (!IS_ERR(vma) || PTR_ERR(vma) != -EAGAIN)
		return vma;

(the !IS_ERR() bit is probably a bit redundant but makes things clearer
vs. just the PTR_ERR() thing)

Then do the rest below.


> +		lock_ctx->locked_vma = vma;
> +	} else if (PTR_ERR(vma) == -EAGAIN) {
> +		/* Fallback to mmap_lock on vma->vm_refcnt overflow */
> +		mmap_read_lock(lock_ctx->mm);
> +		vma = find_vma(lock_ctx->mm, addr);
> +		lock_ctx->mmap_locked = true;

Kinda sucks we have two separate ways of doing fallback now, this
open-coded thing and fallback_to_mmap_lock().

Sort of hard to combine since we have subtly diffrent semantics - the RCU
read lock is being held in the /proc/$pid/maps case, but here we've
released it already.

> +	}
> +
> +	return vma;
> +}
> +
> +#else /* CONFIG_PER_VMA_LOCK */
> +
> +static int query_vma_setup(struct proc_maps_locking_ctx *lock_ctx)
> +{
> +	return mmap_read_lock_killable(lock_ctx->mm);
>  }
>
> -static struct vm_area_struct *query_vma_find_by_addr(struct mm_struct *mm, unsigned long addr)
> +static void query_vma_teardown(struct proc_maps_locking_ctx *lock_ctx)
>  {
> -	return find_vma(mm, addr);
> +	mmap_read_unlock(lock_ctx->mm);
>  }
>
> -static struct vm_area_struct *query_matching_vma(struct mm_struct *mm,
> +static struct vm_area_struct *query_vma_find_by_addr(struct proc_maps_locking_ctx *lock_ctx,
> +						     unsigned long addr)
> +{
> +	return find_vma(lock_ctx->mm, addr);
> +}
> +
> +#endif  /* CONFIG_PER_VMA_LOCK */
> +
> +static struct vm_area_struct *query_matching_vma(struct proc_maps_locking_ctx *lock_ctx,
>  						 unsigned long addr, u32 flags)
>  {
>  	struct vm_area_struct *vma;
>
>  next_vma:
> -	vma = query_vma_find_by_addr(mm, addr);
> +	vma = query_vma_find_by_addr(lock_ctx, addr);
> +	if (IS_ERR(vma))
> +		return vma;
> +
>  	if (!vma)
>  		goto no_vma;
>
> @@ -579,11 +632,11 @@ static struct vm_area_struct *query_matching_vma(struct mm_struct *mm,
>  	return ERR_PTR(-ENOENT);
>  }
>
> -static int do_procmap_query(struct proc_maps_private *priv, void __user *uarg)
> +static int do_procmap_query(struct mm_struct *mm, void __user *uarg)
>  {
> +	struct proc_maps_locking_ctx lock_ctx = { .mm = mm };

>  	struct procmap_query karg;
>  	struct vm_area_struct *vma;
> -	struct mm_struct *mm;
>  	const char *name = NULL;
>  	char build_id_buf[BUILD_ID_SIZE_MAX], *name_buf = NULL;
>  	__u64 usize;
> @@ -610,17 +663,16 @@ static int do_procmap_query(struct proc_maps_private *priv, void __user *uarg)
>  	if (!!karg.build_id_size != !!karg.build_id_addr)
>  		return -EINVAL;
>
> -	mm = priv->lock_ctx.mm;
>  	if (!mm || !mmget_not_zero(mm))
>  		return -ESRCH;
>
> -	err = query_vma_setup(mm);
> +	err = query_vma_setup(&lock_ctx);
>  	if (err) {
>  		mmput(mm);
>  		return err;
>  	}
>
> -	vma = query_matching_vma(mm, karg.query_addr, karg.query_flags);
> +	vma = query_matching_vma(&lock_ctx, karg.query_addr, karg.query_flags);
>  	if (IS_ERR(vma)) {
>  		err = PTR_ERR(vma);
>  		vma = NULL;
> @@ -705,7 +757,7 @@ static int do_procmap_query(struct proc_maps_private *priv, void __user *uarg)
>  	}
>
>  	/* unlock vma or mmap_lock, and put mm_struct before copying data to user */
> -	query_vma_teardown(mm, vma);
> +	query_vma_teardown(&lock_ctx);
>  	mmput(mm);
>
>  	if (karg.vma_name_size && copy_to_user(u64_to_user_ptr(karg.vma_name_addr),
> @@ -725,7 +777,7 @@ static int do_procmap_query(struct proc_maps_private *priv, void __user *uarg)
>  	return 0;
>
>  out:
> -	query_vma_teardown(mm, vma);
> +	query_vma_teardown(&lock_ctx);
>  	mmput(mm);
>  	kfree(name_buf);
>  	return err;
> @@ -738,7 +790,7 @@ static long procfs_procmap_ioctl(struct file *file, unsigned int cmd, unsigned l
>
>  	switch (cmd) {
>  	case PROCMAP_QUERY:
> -		return do_procmap_query(priv, (void __user *)arg);
> +		return do_procmap_query(priv->lock_ctx.mm, (void __user *)arg);

OK this confused me until I worked it through.

We set priv->lock_ctx.mm in:

pid_maps_open() -> do_maps_open() -> proc_maps_open()

Which it gets from proc_mem_open() which figures out the mm.

Maybe one for 2/3, but it'd be nice to have a comment saying something
about how this is set, since it being part of lock_ctx makes it seem like
it's something that would be set elsewhere.

Since we have fallback stuff and want to thread through this new lokc
context type I guess it makes sense to put it here but given that's the
case, let's maybe just add a comment here to clarify.

>  	default:
>  		return -ENOIOCTLCMD;
>  	}
> --
> 2.50.1.565.gc32cd1483b-goog
>

Cheers, Lorenzo

On Wed, Aug 6, 2025 at 12:03 PM Lorenzo Stoakes
<lorenzo.stoakes@oracle.com> wrote:
>
> On Wed, Aug 06, 2025 at 08:59:04AM -0700, Suren Baghdasaryan wrote:
> > Utilize per-vma locks to stabilize vma after lookup without taking
> > mmap_lock during PROCMAP_QUERY ioctl execution. If vma lock is
> > contended, we fall back to mmap_lock but take it only momentarily
> > to lock the vma and release the mmap_lock. In a very unlikely case
> > of vm_refcnt overflow, this fall back path will fail and ioctl is
> > done under mmap_lock protection.
> >
> > This change is designed to reduce mmap_lock contention and prevent
> > PROCMAP_QUERY ioctl calls from blocking address space updates.
> >
> > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
>
> A lot of nits but nothing's really standing out as broken, AFAICT...
>
> > ---
> >  fs/proc/task_mmu.c | 84 +++++++++++++++++++++++++++++++++++++---------
> >  1 file changed, 68 insertions(+), 16 deletions(-)
> >
> > diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> > index 45134335e086..0396315dfaee 100644
> > --- a/fs/proc/task_mmu.c
> > +++ b/fs/proc/task_mmu.c
> > @@ -517,28 +517,81 @@ static int pid_maps_open(struct inode *inode, struct file *file)
> >               PROCMAP_QUERY_VMA_FLAGS                         \
> >  )
> >
> > -static int query_vma_setup(struct mm_struct *mm)
> > +#ifdef CONFIG_PER_VMA_LOCK
> > +
> > +static int query_vma_setup(struct proc_maps_locking_ctx *lock_ctx)
> >  {
> > -     return mmap_read_lock_killable(mm);
> > +     lock_ctx->locked_vma = NULL;
> > +     lock_ctx->mmap_locked = false;
>
> We also do this in lock_vma_range(), seems sensible to factor out? E.g.:
>
> static void ctx_clear_vma(struct proc_maps_locking_ctx *lock_ctx)

That name really confused me :) Maybe lock_vma_ctx_init() or something
along these lines. If we can't think of a good name I would prefer to
keep it as is, given it's only two lines and used only in two places.

> {
>         lock_ctx->locked_vma = NULL;
>         lock_ctx->mmap_locked = false;
> }
>
> > +
> > +     return 0;
> >  }
> >
> > -static void query_vma_teardown(struct mm_struct *mm, struct vm_area_struct *vma)
> > +static void query_vma_teardown(struct proc_maps_locking_ctx *lock_ctx)
> >  {
> > -     mmap_read_unlock(mm);
> > +     if (lock_ctx->mmap_locked)
> > +             mmap_read_unlock(lock_ctx->mm);
>
> Maybe worth a comment as to why we leave lock_ctx->mmap_locked set here?

Sure. The reason is that this is a teardown stage and lock_ctx won't
be used anymore. I guess I could reset it just to leave lock_ctx
consistent instead of adding a comment. Will do that.

>
> > +     else
> > +             unlock_vma(lock_ctx);
>
> Should have said on 2/3, but I wonder if we should prefix with ctx_, as
> 'unlock_vma()' and 'lock_vma()' seem like core functions... esp. since we
> have vma_start_read/write() rather than functions that reference locking.
>
> So - ctx_unlock_vma() and ctx_lock_vma() or unlock_ctx_vma() /
> lock_ctx_vma()?

unlock_ctx_vma() / lock_ctx_vma() sounds good to me.

>
> > +}
> > +
> > +static struct vm_area_struct *query_vma_find_by_addr(struct proc_maps_locking_ctx *lock_ctx,
> > +                                                  unsigned long addr)
> > +{
> > +     struct vm_area_struct *vma;
> > +     struct vma_iterator vmi;
> > +
> > +     if (lock_ctx->mmap_locked)
> > +             return find_vma(lock_ctx->mm, addr);
> > +
> > +     unlock_vma(lock_ctx);
> > +     rcu_read_lock();
> > +     vma_iter_init(&vmi, lock_ctx->mm, addr);
> > +     vma = lock_next_vma(lock_ctx->mm, &vmi, addr);
> > +     rcu_read_unlock();
>
> I think a comment at the top of this block would be useful, something like
> 'We unlock any previously locked VMA and find the next under RCU'.

Ack.

>
> > +
> > +     if (!IS_ERR_OR_NULL(vma)) {
>
> Is the NULL bit here really necessary? presumably lock_ctx->locked_vma is
> expected to be NULL already, so we're not overwriting anything here.
>
> In fact we could get rid of the horrid if/else here with a guard clause like:
>
>         if (!IS_ERR(vma) || PTR_ERR(vma) != -EAGAIN)
>                 return vma;

We still need to assign lock_ctx->locked_vma when !IS_ERR(vma) before
we return the vma, so the lines about would not be correct. I can
change it to:

if (!vma)
    return NULL;

if (!IS_ERR(vma)) {
    lock_ctx->locked_vma = vma;
    return vma;
}

if (PTR_ERR(vma) == -EAGAIN) {
    /* Fallback to mmap_lock on vma->vm_refcnt overflow */
    ...
}
return vma;

I think that would be the equivalent of what I currently have. Would
you prefer that?

>
> (the !IS_ERR() bit is probably a bit redundant but makes things clearer
> vs. just the PTR_ERR() thing)
>
> Then do the rest below.
>
>
> > +             lock_ctx->locked_vma = vma;
> > +     } else if (PTR_ERR(vma) == -EAGAIN) {
> > +             /* Fallback to mmap_lock on vma->vm_refcnt overflow */
> > +             mmap_read_lock(lock_ctx->mm);
> > +             vma = find_vma(lock_ctx->mm, addr);
> > +             lock_ctx->mmap_locked = true;
>
> Kinda sucks we have two separate ways of doing fallback now, this
> open-coded thing and fallback_to_mmap_lock().
>
> Sort of hard to combine since we have subtly diffrent semantics - the RCU
> read lock is being held in the /proc/$pid/maps case, but here we've
> released it already.

Yeah, plus that one uses iterators and this one doesn't... I don't
think it's worth trying to shoehorn them together given that the code
is quite short.

>
> > +     }
> > +
> > +     return vma;
> > +}
> > +
> > +#else /* CONFIG_PER_VMA_LOCK */
> > +
> > +static int query_vma_setup(struct proc_maps_locking_ctx *lock_ctx)
> > +{
> > +     return mmap_read_lock_killable(lock_ctx->mm);
> >  }
> >
> > -static struct vm_area_struct *query_vma_find_by_addr(struct mm_struct *mm, unsigned long addr)
> > +static void query_vma_teardown(struct proc_maps_locking_ctx *lock_ctx)
> >  {
> > -     return find_vma(mm, addr);
> > +     mmap_read_unlock(lock_ctx->mm);
> >  }
> >
> > -static struct vm_area_struct *query_matching_vma(struct mm_struct *mm,
> > +static struct vm_area_struct *query_vma_find_by_addr(struct proc_maps_locking_ctx *lock_ctx,
> > +                                                  unsigned long addr)
> > +{
> > +     return find_vma(lock_ctx->mm, addr);
> > +}
> > +
> > +#endif  /* CONFIG_PER_VMA_LOCK */
> > +
> > +static struct vm_area_struct *query_matching_vma(struct proc_maps_locking_ctx *lock_ctx,
> >                                                unsigned long addr, u32 flags)
> >  {
> >       struct vm_area_struct *vma;
> >
> >  next_vma:
> > -     vma = query_vma_find_by_addr(mm, addr);
> > +     vma = query_vma_find_by_addr(lock_ctx, addr);
> > +     if (IS_ERR(vma))
> > +             return vma;
> > +
> >       if (!vma)
> >               goto no_vma;
> >
> > @@ -579,11 +632,11 @@ static struct vm_area_struct *query_matching_vma(struct mm_struct *mm,
> >       return ERR_PTR(-ENOENT);
> >  }
> >
> > -static int do_procmap_query(struct proc_maps_private *priv, void __user *uarg)
> > +static int do_procmap_query(struct mm_struct *mm, void __user *uarg)
> >  {
> > +     struct proc_maps_locking_ctx lock_ctx = { .mm = mm };
>
> >       struct procmap_query karg;
> >       struct vm_area_struct *vma;
> > -     struct mm_struct *mm;
> >       const char *name = NULL;
> >       char build_id_buf[BUILD_ID_SIZE_MAX], *name_buf = NULL;
> >       __u64 usize;
> > @@ -610,17 +663,16 @@ static int do_procmap_query(struct proc_maps_private *priv, void __user *uarg)
> >       if (!!karg.build_id_size != !!karg.build_id_addr)
> >               return -EINVAL;
> >
> > -     mm = priv->lock_ctx.mm;
> >       if (!mm || !mmget_not_zero(mm))
> >               return -ESRCH;
> >
> > -     err = query_vma_setup(mm);
> > +     err = query_vma_setup(&lock_ctx);
> >       if (err) {
> >               mmput(mm);
> >               return err;
> >       }
> >
> > -     vma = query_matching_vma(mm, karg.query_addr, karg.query_flags);
> > +     vma = query_matching_vma(&lock_ctx, karg.query_addr, karg.query_flags);
> >       if (IS_ERR(vma)) {
> >               err = PTR_ERR(vma);
> >               vma = NULL;
> > @@ -705,7 +757,7 @@ static int do_procmap_query(struct proc_maps_private *priv, void __user *uarg)
> >       }
> >
> >       /* unlock vma or mmap_lock, and put mm_struct before copying data to user */
> > -     query_vma_teardown(mm, vma);
> > +     query_vma_teardown(&lock_ctx);
> >       mmput(mm);
> >
> >       if (karg.vma_name_size && copy_to_user(u64_to_user_ptr(karg.vma_name_addr),
> > @@ -725,7 +777,7 @@ static int do_procmap_query(struct proc_maps_private *priv, void __user *uarg)
> >       return 0;
> >
> >  out:
> > -     query_vma_teardown(mm, vma);
> > +     query_vma_teardown(&lock_ctx);
> >       mmput(mm);
> >       kfree(name_buf);
> >       return err;
> > @@ -738,7 +790,7 @@ static long procfs_procmap_ioctl(struct file *file, unsigned int cmd, unsigned l
> >
> >       switch (cmd) {
> >       case PROCMAP_QUERY:
> > -             return do_procmap_query(priv, (void __user *)arg);
> > +             return do_procmap_query(priv->lock_ctx.mm, (void __user *)arg);
>
> OK this confused me until I worked it through.
>
> We set priv->lock_ctx.mm in:
>
> pid_maps_open() -> do_maps_open() -> proc_maps_open()
>
> Which it gets from proc_mem_open() which figures out the mm.
>
> Maybe one for 2/3, but it'd be nice to have a comment saying something
> about how this is set, since it being part of lock_ctx makes it seem like
> it's something that would be set elsewhere.
>
> Since we have fallback stuff and want to thread through this new lokc
> context type I guess it makes sense to put it here but given that's the
> case, let's maybe just add a comment here to clarify.

Ok, something like "lock_ctx.mm is set during file open operation" ?

>
> >       default:
> >               return -ENOIOCTLCMD;
> >       }
> > --
> > 2.50.1.565.gc32cd1483b-goog
> >
>
> Cheers, Lorenzo

On Wed, Aug 06, 2025 at 02:46:00PM -0700, Suren Baghdasaryan wrote:
> On Wed, Aug 6, 2025 at 12:03 PM Lorenzo Stoakes
> <lorenzo.stoakes@oracle.com> wrote:
> >
> > On Wed, Aug 06, 2025 at 08:59:04AM -0700, Suren Baghdasaryan wrote:
> > > Utilize per-vma locks to stabilize vma after lookup without taking
> > > mmap_lock during PROCMAP_QUERY ioctl execution. If vma lock is
> > > contended, we fall back to mmap_lock but take it only momentarily
> > > to lock the vma and release the mmap_lock. In a very unlikely case
> > > of vm_refcnt overflow, this fall back path will fail and ioctl is
> > > done under mmap_lock protection.
> > >
> > > This change is designed to reduce mmap_lock contention and prevent
> > > PROCMAP_QUERY ioctl calls from blocking address space updates.
> > >
> > > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> >
> > A lot of nits but nothing's really standing out as broken, AFAICT...
> >
> > > ---
> > >  fs/proc/task_mmu.c | 84 +++++++++++++++++++++++++++++++++++++---------
> > >  1 file changed, 68 insertions(+), 16 deletions(-)
> > >
> > > diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> > > index 45134335e086..0396315dfaee 100644
> > > --- a/fs/proc/task_mmu.c
> > > +++ b/fs/proc/task_mmu.c
> > > @@ -517,28 +517,81 @@ static int pid_maps_open(struct inode *inode, struct file *file)
> > >               PROCMAP_QUERY_VMA_FLAGS                         \
> > >  )
> > >
> > > -static int query_vma_setup(struct mm_struct *mm)
> > > +#ifdef CONFIG_PER_VMA_LOCK
> > > +
> > > +static int query_vma_setup(struct proc_maps_locking_ctx *lock_ctx)
> > >  {
> > > -     return mmap_read_lock_killable(mm);
> > > +     lock_ctx->locked_vma = NULL;
> > > +     lock_ctx->mmap_locked = false;
> >
> > We also do this in lock_vma_range(), seems sensible to factor out? E.g.:
> >
> > static void ctx_clear_vma(struct proc_maps_locking_ctx *lock_ctx)
>
> That name really confused me :) Maybe lock_vma_ctx_init() or something
> along these lines. If we can't think of a good name I would prefer to
> keep it as is, given it's only two lines and used only in two places.

Yeah you're right, that name isn't great (it's hard to get naming right
isn't it? :P)

I think it's worth separating out just because I find this:

helper_struct->field1 = X;
helper_struct->field2 = Y;

Open-coding fiddly and prone to error, what if you add a new field later
etc.

It's also semantically useful to know that updating one field impliles the
update of another.

>
> > {
> >         lock_ctx->locked_vma = NULL;
> >         lock_ctx->mmap_locked = false;
> > }
> >
> > > +
> > > +     return 0;
> > >  }
> > >
> > > -static void query_vma_teardown(struct mm_struct *mm, struct vm_area_struct *vma)
> > > +static void query_vma_teardown(struct proc_maps_locking_ctx *lock_ctx)
> > >  {
> > > -     mmap_read_unlock(mm);
> > > +     if (lock_ctx->mmap_locked)
> > > +             mmap_read_unlock(lock_ctx->mm);
> >
> > Maybe worth a comment as to why we leave lock_ctx->mmap_locked set here?
>
> Sure. The reason is that this is a teardown stage and lock_ctx won't
> be used anymore. I guess I could reset it just to leave lock_ctx
> consistent instead of adding a comment. Will do that.

Thanks makes sense.

>
> >
> > > +     else
> > > +             unlock_vma(lock_ctx);
> >
> > Should have said on 2/3, but I wonder if we should prefix with ctx_, as
> > 'unlock_vma()' and 'lock_vma()' seem like core functions... esp. since we
> > have vma_start_read/write() rather than functions that reference locking.
> >
> > So - ctx_unlock_vma() and ctx_lock_vma() or unlock_ctx_vma() /
> > lock_ctx_vma()?
>
> unlock_ctx_vma() / lock_ctx_vma() sounds good to me.
>
> >
> > > +}
> > > +
> > > +static struct vm_area_struct *query_vma_find_by_addr(struct proc_maps_locking_ctx *lock_ctx,
> > > +                                                  unsigned long addr)
> > > +{
> > > +     struct vm_area_struct *vma;
> > > +     struct vma_iterator vmi;
> > > +
> > > +     if (lock_ctx->mmap_locked)
> > > +             return find_vma(lock_ctx->mm, addr);
> > > +
> > > +     unlock_vma(lock_ctx);
> > > +     rcu_read_lock();
> > > +     vma_iter_init(&vmi, lock_ctx->mm, addr);
> > > +     vma = lock_next_vma(lock_ctx->mm, &vmi, addr);
> > > +     rcu_read_unlock();
> >
> > I think a comment at the top of this block would be useful, something like
> > 'We unlock any previously locked VMA and find the next under RCU'.
>
> Ack.
>
> >
> > > +
> > > +     if (!IS_ERR_OR_NULL(vma)) {
> >
> > Is the NULL bit here really necessary? presumably lock_ctx->locked_vma is
> > expected to be NULL already, so we're not overwriting anything here.
> >
> > In fact we could get rid of the horrid if/else here with a guard clause like:
> >
> >         if (!IS_ERR(vma) || PTR_ERR(vma) != -EAGAIN)
> >                 return vma;
>
> We still need to assign lock_ctx->locked_vma when !IS_ERR(vma) before
> we return the vma, so the lines about would not be correct. I can
> change it to:
>
> if (!vma)
>     return NULL;
>
> if (!IS_ERR(vma)) {
>     lock_ctx->locked_vma = vma;
>     return vma;
> }
>
> if (PTR_ERR(vma) == -EAGAIN) {
>     /* Fallback to mmap_lock on vma->vm_refcnt overflow */
>     ...
> }
> return vma;
>
> I think that would be the equivalent of what I currently have. Would
> you prefer that?

Yeah sorry, sort of sketching this out quickly here.

Yeah what you suggest looks good thanks!

>
> >
> > (the !IS_ERR() bit is probably a bit redundant but makes things clearer
> > vs. just the PTR_ERR() thing)
> >
> > Then do the rest below.
> >
> >
> > > +             lock_ctx->locked_vma = vma;
> > > +     } else if (PTR_ERR(vma) == -EAGAIN) {
> > > +             /* Fallback to mmap_lock on vma->vm_refcnt overflow */
> > > +             mmap_read_lock(lock_ctx->mm);
> > > +             vma = find_vma(lock_ctx->mm, addr);
> > > +             lock_ctx->mmap_locked = true;
> >
> > Kinda sucks we have two separate ways of doing fallback now, this
> > open-coded thing and fallback_to_mmap_lock().
> >
> > Sort of hard to combine since we have subtly diffrent semantics - the RCU
> > read lock is being held in the /proc/$pid/maps case, but here we've
> > released it already.
>
> Yeah, plus that one uses iterators and this one doesn't... I don't
> think it's worth trying to shoehorn them together given that the code
> is quite short.

Yeah right.

I sort of wish we could have things be a little more consistent across the
two, but I think that would need to be part of a refactoring of this code
in general, so is not really relevant here.

So leave it as-is for now that's fine!

>
> >
> > > +     }
> > > +
> > > +     return vma;
> > > +}
> > > +
> > > +#else /* CONFIG_PER_VMA_LOCK */
> > > +
> > > +static int query_vma_setup(struct proc_maps_locking_ctx *lock_ctx)
> > > +{
> > > +     return mmap_read_lock_killable(lock_ctx->mm);
> > >  }
> > >
> > > -static struct vm_area_struct *query_vma_find_by_addr(struct mm_struct *mm, unsigned long addr)
> > > +static void query_vma_teardown(struct proc_maps_locking_ctx *lock_ctx)
> > >  {
> > > -     return find_vma(mm, addr);
> > > +     mmap_read_unlock(lock_ctx->mm);
> > >  }
> > >
> > > -static struct vm_area_struct *query_matching_vma(struct mm_struct *mm,
> > > +static struct vm_area_struct *query_vma_find_by_addr(struct proc_maps_locking_ctx *lock_ctx,
> > > +                                                  unsigned long addr)
> > > +{
> > > +     return find_vma(lock_ctx->mm, addr);
> > > +}
> > > +
> > > +#endif  /* CONFIG_PER_VMA_LOCK */
> > > +
> > > +static struct vm_area_struct *query_matching_vma(struct proc_maps_locking_ctx *lock_ctx,
> > >                                                unsigned long addr, u32 flags)
> > >  {
> > >       struct vm_area_struct *vma;
> > >
> > >  next_vma:
> > > -     vma = query_vma_find_by_addr(mm, addr);
> > > +     vma = query_vma_find_by_addr(lock_ctx, addr);
> > > +     if (IS_ERR(vma))
> > > +             return vma;
> > > +
> > >       if (!vma)
> > >               goto no_vma;
> > >
> > > @@ -579,11 +632,11 @@ static struct vm_area_struct *query_matching_vma(struct mm_struct *mm,
> > >       return ERR_PTR(-ENOENT);
> > >  }
> > >
> > > -static int do_procmap_query(struct proc_maps_private *priv, void __user *uarg)
> > > +static int do_procmap_query(struct mm_struct *mm, void __user *uarg)
> > >  {
> > > +     struct proc_maps_locking_ctx lock_ctx = { .mm = mm };
> >
> > >       struct procmap_query karg;
> > >       struct vm_area_struct *vma;
> > > -     struct mm_struct *mm;
> > >       const char *name = NULL;
> > >       char build_id_buf[BUILD_ID_SIZE_MAX], *name_buf = NULL;
> > >       __u64 usize;
> > > @@ -610,17 +663,16 @@ static int do_procmap_query(struct proc_maps_private *priv, void __user *uarg)
> > >       if (!!karg.build_id_size != !!karg.build_id_addr)
> > >               return -EINVAL;
> > >
> > > -     mm = priv->lock_ctx.mm;
> > >       if (!mm || !mmget_not_zero(mm))
> > >               return -ESRCH;
> > >
> > > -     err = query_vma_setup(mm);
> > > +     err = query_vma_setup(&lock_ctx);
> > >       if (err) {
> > >               mmput(mm);
> > >               return err;
> > >       }
> > >
> > > -     vma = query_matching_vma(mm, karg.query_addr, karg.query_flags);
> > > +     vma = query_matching_vma(&lock_ctx, karg.query_addr, karg.query_flags);
> > >       if (IS_ERR(vma)) {
> > >               err = PTR_ERR(vma);
> > >               vma = NULL;
> > > @@ -705,7 +757,7 @@ static int do_procmap_query(struct proc_maps_private *priv, void __user *uarg)
> > >       }
> > >
> > >       /* unlock vma or mmap_lock, and put mm_struct before copying data to user */
> > > -     query_vma_teardown(mm, vma);
> > > +     query_vma_teardown(&lock_ctx);
> > >       mmput(mm);
> > >
> > >       if (karg.vma_name_size && copy_to_user(u64_to_user_ptr(karg.vma_name_addr),
> > > @@ -725,7 +777,7 @@ static int do_procmap_query(struct proc_maps_private *priv, void __user *uarg)
> > >       return 0;
> > >
> > >  out:
> > > -     query_vma_teardown(mm, vma);
> > > +     query_vma_teardown(&lock_ctx);
> > >       mmput(mm);
> > >       kfree(name_buf);
> > >       return err;
> > > @@ -738,7 +790,7 @@ static long procfs_procmap_ioctl(struct file *file, unsigned int cmd, unsigned l
> > >
> > >       switch (cmd) {
> > >       case PROCMAP_QUERY:
> > > -             return do_procmap_query(priv, (void __user *)arg);
> > > +             return do_procmap_query(priv->lock_ctx.mm, (void __user *)arg);
> >
> > OK this confused me until I worked it through.
> >
> > We set priv->lock_ctx.mm in:
> >
> > pid_maps_open() -> do_maps_open() -> proc_maps_open()
> >
> > Which it gets from proc_mem_open() which figures out the mm.
> >
> > Maybe one for 2/3, but it'd be nice to have a comment saying something
> > about how this is set, since it being part of lock_ctx makes it seem like
> > it's something that would be set elsewhere.
> >
> > Since we have fallback stuff and want to thread through this new lokc
> > context type I guess it makes sense to put it here but given that's the
> > case, let's maybe just add a comment here to clarify.
>
> Ok, something like "lock_ctx.mm is set during file open operation" ?

Yeah that's fine thanks!

>
> >
> > >       default:
> > >               return -ENOIOCTLCMD;
> > >       }
> > > --
> > > 2.50.1.565.gc32cd1483b-goog
> > >
> >
> > Cheers, Lorenzo

On Wed,  6 Aug 2025 08:59:04 -0700 Suren Baghdasaryan <surenb@google.com> wrote:

> Utilize per-vma locks to stabilize vma after lookup without taking
> mmap_lock during PROCMAP_QUERY ioctl execution. If vma lock is
> contended, we fall back to mmap_lock but take it only momentarily
> to lock the vma and release the mmap_lock. In a very unlikely case
> of vm_refcnt overflow, this fall back path will fail and ioctl is
> done under mmap_lock protection.
> 
> This change is designed to reduce mmap_lock contention and prevent
> PROCMAP_QUERY ioctl calls from blocking address space updates.
> 
> Signed-off-by: Suren Baghdasaryan <surenb@google.com>

Acked-by: SeongJae Park <sj@kernel.org>


Thanks,
SJ

[...]