From: David Hildenbrand <david@redhat.com>
Let's allow for making MADV_COLLAPSE succeed on areas that neither have
VM_HUGEPAGE nor VM_NOHUGEPAGE when we have THP disabled
unless explicitly advised (PR_THP_DISABLE_EXCEPT_ADVISED).
MADV_COLLAPSE is a clear advise that we want to collapse.
Note that we still respect the VM_NOHUGEPAGE flag, just like
MADV_COLLAPSE always does. So consequently, MADV_COLLAPSE is now only
refused on VM_NOHUGEPAGE with PR_THP_DISABLE_EXCEPT_ADVISED.
Co-developed-by: Usama Arif <usamaarif642@gmail.com>
Signed-off-by: Usama Arif <usamaarif642@gmail.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
---
include/linux/huge_mm.h | 8 +++++++-
include/uapi/linux/prctl.h | 2 +-
mm/huge_memory.c | 5 +++--
mm/memory.c | 6 ++++--
mm/shmem.c | 2 +-
5 files changed, 16 insertions(+), 7 deletions(-)
diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index b0ff54eee81c..aeaf93f8ac2e 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -329,7 +329,7 @@ struct thpsize {
* through madvise or prctl.
*/
static inline bool vma_thp_disabled(struct vm_area_struct *vma,
- vm_flags_t vm_flags)
+ vm_flags_t vm_flags, bool forced_collapse)
{
/* Are THPs disabled for this VMA? */
if (vm_flags & VM_NOHUGEPAGE)
@@ -343,6 +343,12 @@ static inline bool vma_thp_disabled(struct vm_area_struct *vma,
*/
if (vm_flags & VM_HUGEPAGE)
return false;
+ /*
+ * Forcing a collapse (e.g., madv_collapse), is a clear advise to
+ * use THPs.
+ */
+ if (forced_collapse)
+ return false;
return test_bit(MMF_DISABLE_THP_EXCEPT_ADVISED, &vma->vm_mm->flags);
}
diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
index 9c1d6e49b8a9..ee4165738779 100644
--- a/include/uapi/linux/prctl.h
+++ b/include/uapi/linux/prctl.h
@@ -185,7 +185,7 @@ struct prctl_mm_map {
#define PR_SET_THP_DISABLE 41
/*
* Don't disable THPs when explicitly advised (e.g., MADV_HUGEPAGE /
- * VM_HUGEPAGE).
+ * VM_HUGEPAGE / MADV_COLLAPSE).
*/
# define PR_THP_DISABLE_EXCEPT_ADVISED (1 << 1)
#define PR_GET_THP_DISABLE 42
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 85252b468f80..ef5ccb0ec5d5 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -104,7 +104,8 @@ unsigned long __thp_vma_allowable_orders(struct vm_area_struct *vma,
{
const bool smaps = type == TVA_SMAPS;
const bool in_pf = type == TVA_PAGEFAULT;
- const bool enforce_sysfs = type != TVA_FORCED_COLLAPSE;
+ const bool forced_collapse = type == TVA_FORCED_COLLAPSE;
+ const bool enforce_sysfs = !forced_collapse;
unsigned long supported_orders;
/* Check the intersection of requested and supported orders. */
@@ -122,7 +123,7 @@ unsigned long __thp_vma_allowable_orders(struct vm_area_struct *vma,
if (!vma->vm_mm) /* vdso */
return 0;
- if (thp_disabled_by_hw() || vma_thp_disabled(vma, vm_flags))
+ if (thp_disabled_by_hw() || vma_thp_disabled(vma, vm_flags, forced_collapse))
return 0;
/* khugepaged doesn't collapse DAX vma, but page fault is fine. */
diff --git a/mm/memory.c b/mm/memory.c
index be761753f240..bd04212d6f79 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -5186,9 +5186,11 @@ vm_fault_t do_set_pmd(struct vm_fault *vmf, struct folio *folio, struct page *pa
* It is too late to allocate a small folio, we already have a large
* folio in the pagecache: especially s390 KVM cannot tolerate any
* PMD mappings, but PTE-mapped THP are fine. So let's simply refuse any
- * PMD mappings if THPs are disabled.
+ * PMD mappings if THPs are disabled. As we already have a THP ...
+ * behave as if we are forcing a collapse.
*/
- if (thp_disabled_by_hw() || vma_thp_disabled(vma, vma->vm_flags))
+ if (thp_disabled_by_hw() || vma_thp_disabled(vma, vma->vm_flags,
+ /* forced_collapse=*/ true))
return ret;
if (!thp_vma_suitable_order(vma, haddr, PMD_ORDER))
diff --git a/mm/shmem.c b/mm/shmem.c
index e6cdfda08aed..30609197a266 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1816,7 +1816,7 @@ unsigned long shmem_allowable_huge_orders(struct inode *inode,
vm_flags_t vm_flags = vma ? vma->vm_flags : 0;
unsigned int global_orders;
- if (thp_disabled_by_hw() || (vma && vma_thp_disabled(vma, vm_flags)))
+ if (thp_disabled_by_hw() || (vma && vma_thp_disabled(vma, vm_flags, shmem_huge_force)))
return 0;
global_orders = shmem_huge_global_enabled(inode, index, write_end,
--
2.47.3Nits on subject:
- It's >75 chars
- advise is the verb, advice is the noun.
On Thu, Jul 31, 2025 at 01:27:20PM +0100, Usama Arif wrote:
> From: David Hildenbrand <david@redhat.com>
>
> Let's allow for making MADV_COLLAPSE succeed on areas that neither have
> VM_HUGEPAGE nor VM_NOHUGEPAGE when we have THP disabled
> unless explicitly advised (PR_THP_DISABLE_EXCEPT_ADVISED).
Hmm, I'm not sure about this.
So far this prctl() has been the only way to override MADV_COLLAPSE
behaviour, but now we're allowing for this one case to not.
I suppose the precedent is that MADV_COLLAPSE overrides 'madvise' sysfs
behaviour.
I suppose what saves us here is 'advised' can be read to mean either
MADV_HUGEPAGE or MADV_COLLAPSE.
And yes, MADV_COLLAPSE is clearly the user requesting this behaviour.
I think the vagueness here is one that already existed, because one could
perfectly one have expected MADV_COLLAPSE to obey sysfs and require
MADV_HUGEPAGE to have been applied, but of course this is not the case.
OK so fine.
BUT.
I think the MADV_COLLAPSE man page will need to be updated to mention this.
And I REALLY think we should update the THP doc too to mention all these
prctl() modes.
I'm not sure we cover that right now _at all_ and obviously we should
describe the new flags.
Usama - can you add a patch to this series to do that?
>
> MADV_COLLAPSE is a clear advise that we want to collapse.
advise -> advice.
>
> Note that we still respect the VM_NOHUGEPAGE flag, just like
> MADV_COLLAPSE always does. So consequently, MADV_COLLAPSE is now only
> refused on VM_NOHUGEPAGE with PR_THP_DISABLE_EXCEPT_ADVISED.
You also need to mention the shmem change you've made I think.
>
> Co-developed-by: Usama Arif <usamaarif642@gmail.com>
> Signed-off-by: Usama Arif <usamaarif642@gmail.com>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
> include/linux/huge_mm.h | 8 +++++++-
> include/uapi/linux/prctl.h | 2 +-
> mm/huge_memory.c | 5 +++--
> mm/memory.c | 6 ++++--
> mm/shmem.c | 2 +-
> 5 files changed, 16 insertions(+), 7 deletions(-)
>
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index b0ff54eee81c..aeaf93f8ac2e 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -329,7 +329,7 @@ struct thpsize {
> * through madvise or prctl.
> */
> static inline bool vma_thp_disabled(struct vm_area_struct *vma,
> - vm_flags_t vm_flags)
> + vm_flags_t vm_flags, bool forced_collapse)
> {
> /* Are THPs disabled for this VMA? */
> if (vm_flags & VM_NOHUGEPAGE)
> @@ -343,6 +343,12 @@ static inline bool vma_thp_disabled(struct vm_area_struct *vma,
> */
> if (vm_flags & VM_HUGEPAGE)
> return false;
> + /*
> + * Forcing a collapse (e.g., madv_collapse), is a clear advise to
advise -> advice.
> + * use THPs.
> + */
> + if (forced_collapse)
> + return false;
> return test_bit(MMF_DISABLE_THP_EXCEPT_ADVISED, &vma->vm_mm->flags);
> }
>
> diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
> index 9c1d6e49b8a9..ee4165738779 100644
> --- a/include/uapi/linux/prctl.h
> +++ b/include/uapi/linux/prctl.h
> @@ -185,7 +185,7 @@ struct prctl_mm_map {
> #define PR_SET_THP_DISABLE 41
> /*
> * Don't disable THPs when explicitly advised (e.g., MADV_HUGEPAGE /
> - * VM_HUGEPAGE).
> + * VM_HUGEPAGE / MADV_COLLAPSE).
This is confusing you're mixing VMA flags with MADV ones... maybe just
stick to madvise ones, or add extra context around VM_HUGEPAGE bit?
Would need to be fixed up in a prior commit obviously.
> */
> # define PR_THP_DISABLE_EXCEPT_ADVISED (1 << 1)
> #define PR_GET_THP_DISABLE 42
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 85252b468f80..ef5ccb0ec5d5 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -104,7 +104,8 @@ unsigned long __thp_vma_allowable_orders(struct vm_area_struct *vma,
> {
> const bool smaps = type == TVA_SMAPS;
> const bool in_pf = type == TVA_PAGEFAULT;
> - const bool enforce_sysfs = type != TVA_FORCED_COLLAPSE;
> + const bool forced_collapse = type == TVA_FORCED_COLLAPSE;
> + const bool enforce_sysfs = !forced_collapse;
Can we just get rid of this enforce_sysfs altogether in patch 2/5 and use
forced_collapse?
The first place we use it we negate it:
return orders & shmem_allowable_huge_orders(file_inode(vma->vm_file),
vma, vma->vm_pgoff, 0,
!enforce_sysfs);
And the one other place we'd have to negate, but it actually helps document
behaviour I think.
> unsigned long supported_orders;
>
> /* Check the intersection of requested and supported orders. */
> @@ -122,7 +123,7 @@ unsigned long __thp_vma_allowable_orders(struct vm_area_struct *vma,
> if (!vma->vm_mm) /* vdso */
> return 0;
>
> - if (thp_disabled_by_hw() || vma_thp_disabled(vma, vm_flags))
> + if (thp_disabled_by_hw() || vma_thp_disabled(vma, vm_flags, forced_collapse))
> return 0;
>
> /* khugepaged doesn't collapse DAX vma, but page fault is fine. */
> diff --git a/mm/memory.c b/mm/memory.c
> index be761753f240..bd04212d6f79 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -5186,9 +5186,11 @@ vm_fault_t do_set_pmd(struct vm_fault *vmf, struct folio *folio, struct page *pa
> * It is too late to allocate a small folio, we already have a large
> * folio in the pagecache: especially s390 KVM cannot tolerate any
> * PMD mappings, but PTE-mapped THP are fine. So let's simply refuse any
> - * PMD mappings if THPs are disabled.
> + * PMD mappings if THPs are disabled. As we already have a THP ...
> + * behave as if we are forcing a collapse.
> */
> - if (thp_disabled_by_hw() || vma_thp_disabled(vma, vma->vm_flags))
> + if (thp_disabled_by_hw() || vma_thp_disabled(vma, vma->vm_flags,
> + /* forced_collapse=*/ true))
> return ret;
>
> if (!thp_vma_suitable_order(vma, haddr, PMD_ORDER))
> diff --git a/mm/shmem.c b/mm/shmem.c
> index e6cdfda08aed..30609197a266 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -1816,7 +1816,7 @@ unsigned long shmem_allowable_huge_orders(struct inode *inode,
> vm_flags_t vm_flags = vma ? vma->vm_flags : 0;
> unsigned int global_orders;
>
> - if (thp_disabled_by_hw() || (vma && vma_thp_disabled(vma, vm_flags)))
> + if (thp_disabled_by_hw() || (vma && vma_thp_disabled(vma, vm_flags, shmem_huge_force)))
Hm this is an extra bit of logic, as noted above, we definitely need to
mention we're changing this in the commit message also.
Since shmem_allowable_huge_orders() can only be invoked by
__thp_vma_allowable_orders() with shmem_huge_force set true, and it'd only
be the case should TVA_FORCED_COLLAPSE is set, this makes sense.
> return 0;
>
> global_orders = shmem_huge_global_enabled(inode, index, write_end,
> --
> 2.47.3
>
On 31.07.25 16:38, Lorenzo Stoakes wrote:
> Nits on subject:
>
> - It's >75 chars
No big deal. If we cna come up with something shorter, good.
> - advise is the verb, advice is the noun.
Yeah.
>
> On Thu, Jul 31, 2025 at 01:27:20PM +0100, Usama Arif wrote:
>> From: David Hildenbrand <david@redhat.com>
>>
>> Let's allow for making MADV_COLLAPSE succeed on areas that neither have
>> VM_HUGEPAGE nor VM_NOHUGEPAGE when we have THP disabled
>> unless explicitly advised (PR_THP_DISABLE_EXCEPT_ADVISED).
>
> Hmm, I'm not sure about this.
>
> So far this prctl() has been the only way to override MADV_COLLAPSE
> behaviour, but now we're allowing for this one case to not.
This is not an override really. prctl() disallowed MADV_COLLAPSE, but in
the new mode we don't want that anymore.
> > I suppose the precedent is that MADV_COLLAPSE overrides 'madvise' sysfs
> behaviour.
> > I suppose what saves us here is 'advised' can be read to mean either
> MADV_HUGEPAGE or MADV_COLLAPSE.
> > And yes, MADV_COLLAPSE is clearly the user requesting this behaviour.
Exactly.
>
> I think the vagueness here is one that already existed, because one could
> perfectly one have expected MADV_COLLAPSE to obey sysfs and require
> MADV_HUGEPAGE to have been applied, but of course this is not the case.
Yes.
>
> OK so fine.
>
> BUT.
>
> I think the MADV_COLLAPSE man page will need to be updated to mention this.
>
Yes.
> And I REALLY think we should update the THP doc too to mention all these
> prctl() modes.
>
> I'm not sure we cover that right now _at all_ and obviously we should
> describe the new flags.
>
> Usama - can you add a patch to this series to do that?
Good point, let's document the interaction with prctl().
>
>>
>> MADV_COLLAPSE is a clear advise that we want to collapse.
>
> advise -> advice.
>
>>
>> Note that we still respect the VM_NOHUGEPAGE flag, just like
>> MADV_COLLAPSE always does. So consequently, MADV_COLLAPSE is now only
>> refused on VM_NOHUGEPAGE with PR_THP_DISABLE_EXCEPT_ADVISED.
>
> You also need to mention the shmem change you've made I think.
Yes.
> >>
>> Co-developed-by: Usama Arif <usamaarif642@gmail.com>
>> Signed-off-by: Usama Arif <usamaarif642@gmail.com>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
>> ---
>> include/linux/huge_mm.h | 8 +++++++-
>> include/uapi/linux/prctl.h | 2 +-
>> mm/huge_memory.c | 5 +++--
>> mm/memory.c | 6 ++++--
>> mm/shmem.c | 2 +-
>> 5 files changed, 16 insertions(+), 7 deletions(-)
>>
>> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
>> index b0ff54eee81c..aeaf93f8ac2e 100644
>> --- a/include/linux/huge_mm.h
>> +++ b/include/linux/huge_mm.h
>> @@ -329,7 +329,7 @@ struct thpsize {
>> * through madvise or prctl.
>> */
>> static inline bool vma_thp_disabled(struct vm_area_struct *vma,
>> - vm_flags_t vm_flags)
>> + vm_flags_t vm_flags, bool forced_collapse)
>> {
>> /* Are THPs disabled for this VMA? */
>> if (vm_flags & VM_NOHUGEPAGE)
>> @@ -343,6 +343,12 @@ static inline bool vma_thp_disabled(struct vm_area_struct *vma,
>> */
>> if (vm_flags & VM_HUGEPAGE)
>> return false;
>> + /*
>> + * Forcing a collapse (e.g., madv_collapse), is a clear advise to
>
> advise -> advice.
>
>> + * use THPs.
>> + */
>> + if (forced_collapse)
>> + return false;
>> return test_bit(MMF_DISABLE_THP_EXCEPT_ADVISED, &vma->vm_mm->flags);
>> }
>>
>> diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
>> index 9c1d6e49b8a9..ee4165738779 100644
>> --- a/include/uapi/linux/prctl.h
>> +++ b/include/uapi/linux/prctl.h
>> @@ -185,7 +185,7 @@ struct prctl_mm_map {
>> #define PR_SET_THP_DISABLE 41
>> /*
>> * Don't disable THPs when explicitly advised (e.g., MADV_HUGEPAGE /
>> - * VM_HUGEPAGE).
>> + * VM_HUGEPAGE / MADV_COLLAPSE).
>
> This is confusing you're mixing VMA flags with MADV ones... maybe just
> stick to madvise ones, or add extra context around VM_HUGEPAGE bit?
I don't see anything confusing here, really.
But if it helps you, we can do
(e.g., MADV_HUGEPAGE / VM_HUGEPAGE, MADV_COLLAPSE).
(reason VM_HUGEPAGE is spelled out is that there might be code where we
set VM_HUGEPAGE implicitly in the kernel)
>
> Would need to be fixed up in a prior commit obviously.
>
>> */
>> # define PR_THP_DISABLE_EXCEPT_ADVISED (1 << 1)
>> #define PR_GET_THP_DISABLE 42
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index 85252b468f80..ef5ccb0ec5d5 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -104,7 +104,8 @@ unsigned long __thp_vma_allowable_orders(struct vm_area_struct *vma,
>> {
>> const bool smaps = type == TVA_SMAPS;
>> const bool in_pf = type == TVA_PAGEFAULT;
>> - const bool enforce_sysfs = type != TVA_FORCED_COLLAPSE;
>> + const bool forced_collapse = type == TVA_FORCED_COLLAPSE;
>> + const bool enforce_sysfs = !forced_collapse;
>
> Can we just get rid of this enforce_sysfs altogether in patch 2/5 and use
> forced_collapse?
Let's do that as a separate cleanup on top. I want least churn in that
patch.
(had the same idea while writing that patch, but I have other things to
focus on than cleaning up all this mess)
--
Cheers,
David / dhildenb
On Thu, Jul 31, 2025 at 04:54:38PM +0200, David Hildenbrand wrote:
> On 31.07.25 16:38, Lorenzo Stoakes wrote:
> > Nits on subject:
> >
> > - It's >75 chars
>
> No big deal. If we cna come up with something shorter, good.
>
> > - advise is the verb, advice is the noun.
>
> Yeah.
>
> >
> > On Thu, Jul 31, 2025 at 01:27:20PM +0100, Usama Arif wrote:
> > > From: David Hildenbrand <david@redhat.com>
> > >
> > > Let's allow for making MADV_COLLAPSE succeed on areas that neither have
> > > VM_HUGEPAGE nor VM_NOHUGEPAGE when we have THP disabled
> > > unless explicitly advised (PR_THP_DISABLE_EXCEPT_ADVISED).
> >
> > Hmm, I'm not sure about this.
> >
> > So far this prctl() has been the only way to override MADV_COLLAPSE
> > behaviour, but now we're allowing for this one case to not.
>
> This is not an override really. prctl() disallowed MADV_COLLAPSE, but in the
> new mode we don't want that anymore.
Yeah see below, I sort of convinced myself this is OK.
>
> > > I suppose the precedent is that MADV_COLLAPSE overrides 'madvise' sysfs
> > behaviour.
> > > I suppose what saves us here is 'advised' can be read to mean either
> > MADV_HUGEPAGE or MADV_COLLAPSE.
> > > And yes, MADV_COLLAPSE is clearly the user requesting this behaviour.
>
> Exactly.
And really I guess it's logical right? If you MADV_COLLAPSE you are saying
'I want THPs', if you MADV_HUGEPAGE you are saying 'I want THPs', the
difference being on timing.
>
> >
> > I think the vagueness here is one that already existed, because one could
> > perfectly one have expected MADV_COLLAPSE to obey sysfs and require
> > MADV_HUGEPAGE to have been applied, but of course this is not the case.
>
> Yes.
>
> >
> > OK so fine.
> >
> > BUT.
> >
> > I think the MADV_COLLAPSE man page will need to be updated to mention this.
> >
>
> Yes.
>
> > And I REALLY think we should update the THP doc too to mention all these
> > prctl() modes.
> >
> > I'm not sure we cover that right now _at all_ and obviously we should
> > describe the new flags.
> >
> > Usama - can you add a patch to this series to do that?
>
> Good point, let's document the interaction with prctl().
Thanks, I think we haven't even spoken that much about MADV_COLLAPSE in the
docs, somebody had a patch but then it fizzled out.
But that's a separate task, will add to my TODOs in case nobody else picks
up.
>
> >
> > >
> > > MADV_COLLAPSE is a clear advise that we want to collapse.
> >
> > advise -> advice.
> >
> > >
> > > Note that we still respect the VM_NOHUGEPAGE flag, just like
> > > MADV_COLLAPSE always does. So consequently, MADV_COLLAPSE is now only
> > > refused on VM_NOHUGEPAGE with PR_THP_DISABLE_EXCEPT_ADVISED.
> >
> > You also need to mention the shmem change you've made I think.
>
> Yes.
>
> > >>
> > > Co-developed-by: Usama Arif <usamaarif642@gmail.com>
> > > Signed-off-by: Usama Arif <usamaarif642@gmail.com>
> > > Signed-off-by: David Hildenbrand <david@redhat.com>
> > > ---
> > > include/linux/huge_mm.h | 8 +++++++-
> > > include/uapi/linux/prctl.h | 2 +-
> > > mm/huge_memory.c | 5 +++--
> > > mm/memory.c | 6 ++++--
> > > mm/shmem.c | 2 +-
> > > 5 files changed, 16 insertions(+), 7 deletions(-)
> > >
> > > diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> > > index b0ff54eee81c..aeaf93f8ac2e 100644
> > > --- a/include/linux/huge_mm.h
> > > +++ b/include/linux/huge_mm.h
> > > @@ -329,7 +329,7 @@ struct thpsize {
> > > * through madvise or prctl.
> > > */
> > > static inline bool vma_thp_disabled(struct vm_area_struct *vma,
> > > - vm_flags_t vm_flags)
> > > + vm_flags_t vm_flags, bool forced_collapse)
> > > {
> > > /* Are THPs disabled for this VMA? */
> > > if (vm_flags & VM_NOHUGEPAGE)
> > > @@ -343,6 +343,12 @@ static inline bool vma_thp_disabled(struct vm_area_struct *vma,
> > > */
> > > if (vm_flags & VM_HUGEPAGE)
> > > return false;
> > > + /*
> > > + * Forcing a collapse (e.g., madv_collapse), is a clear advise to
> >
> > advise -> advice.
> >
> > > + * use THPs.
> > > + */
> > > + if (forced_collapse)
> > > + return false;
> > > return test_bit(MMF_DISABLE_THP_EXCEPT_ADVISED, &vma->vm_mm->flags);
> > > }
> > >
> > > diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
> > > index 9c1d6e49b8a9..ee4165738779 100644
> > > --- a/include/uapi/linux/prctl.h
> > > +++ b/include/uapi/linux/prctl.h
> > > @@ -185,7 +185,7 @@ struct prctl_mm_map {
> > > #define PR_SET_THP_DISABLE 41
> > > /*
> > > * Don't disable THPs when explicitly advised (e.g., MADV_HUGEPAGE /
> > > - * VM_HUGEPAGE).
> > > + * VM_HUGEPAGE / MADV_COLLAPSE).
> >
> > This is confusing you're mixing VMA flags with MADV ones... maybe just
> > stick to madvise ones, or add extra context around VM_HUGEPAGE bit?
>
> I don't see anything confusing here, really.
>
> But if it helps you, we can do
> (e.g., MADV_HUGEPAGE / VM_HUGEPAGE, MADV_COLLAPSE).
>
> (reason VM_HUGEPAGE is spelled out is that there might be code where we set
> VM_HUGEPAGE implicitly in the kernel)
Yeah to be clear this is a pretty minor point, and I see why we'd want to
mention VM_HUGEPAGE explicitly, as it is in fact this that overrides THP
being disabled for the process.
>
> >
> > Would need to be fixed up in a prior commit obviously.
> >
> > > */
> > > # define PR_THP_DISABLE_EXCEPT_ADVISED (1 << 1)
> > > #define PR_GET_THP_DISABLE 42
> > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > > index 85252b468f80..ef5ccb0ec5d5 100644
> > > --- a/mm/huge_memory.c
> > > +++ b/mm/huge_memory.c
> > > @@ -104,7 +104,8 @@ unsigned long __thp_vma_allowable_orders(struct vm_area_struct *vma,
> > > {
> > > const bool smaps = type == TVA_SMAPS;
> > > const bool in_pf = type == TVA_PAGEFAULT;
> > > - const bool enforce_sysfs = type != TVA_FORCED_COLLAPSE;
> > > + const bool forced_collapse = type == TVA_FORCED_COLLAPSE;
> > > + const bool enforce_sysfs = !forced_collapse;
> >
> > Can we just get rid of this enforce_sysfs altogether in patch 2/5 and use
> > forced_collapse?
>
> Let's do that as a separate cleanup on top. I want least churn in that
> patch.
>
> (had the same idea while writing that patch, but I have other things to
> focus on than cleaning up all this mess)
Ack, I'm fine with that.
>
> --
> Cheers,
>
> David / dhildenb
>
Cheers, Lorenzo
On 31/07/2025 15:54, David Hildenbrand wrote:
> On 31.07.25 16:38, Lorenzo Stoakes wrote:
>> Nits on subject:
>>
>> - It's >75 chars
>
> No big deal. If we cna come up with something shorter, good.
Changed it to "mm/huge_memory: respect MADV_COLLAPSE with PR_THP_DISABLE_EXCEPT_ADVISED"
for the next revision. That would be 73 chars :)
>
>> - advise is the verb, advice is the noun.
>
> Yeah.
>
>>
>> On Thu, Jul 31, 2025 at 01:27:20PM +0100, Usama Arif wrote:
>>> From: David Hildenbrand <david@redhat.com>
>>>
>>> Let's allow for making MADV_COLLAPSE succeed on areas that neither have
>>> VM_HUGEPAGE nor VM_NOHUGEPAGE when we have THP disabled
>>> unless explicitly advised (PR_THP_DISABLE_EXCEPT_ADVISED).
>>
>> Hmm, I'm not sure about this.
>>
>> So far this prctl() has been the only way to override MADV_COLLAPSE
>> behaviour, but now we're allowing for this one case to not.
>
> This is not an override really. prctl() disallowed MADV_COLLAPSE, but in the new mode we don't want that anymore.
>
>> > I suppose the precedent is that MADV_COLLAPSE overrides 'madvise' sysfs
>> behaviour.
>> > I suppose what saves us here is 'advised' can be read to mean either
>> MADV_HUGEPAGE or MADV_COLLAPSE.
>> > And yes, MADV_COLLAPSE is clearly the user requesting this behaviour.
>
> Exactly.
>
>>
>> I think the vagueness here is one that already existed, because one could
>> perfectly one have expected MADV_COLLAPSE to obey sysfs and require
>> MADV_HUGEPAGE to have been applied, but of course this is not the case.
>
> Yes.
>
>>
>> OK so fine.
>>
>> BUT.
>>
>> I think the MADV_COLLAPSE man page will need to be updated to mention this.
>>
>
> Yes.
>
Thanks, yes will do and send this along with changes to prctl man page after this
makes into mm-stable.
>> And I REALLY think we should update the THP doc too to mention all these
>> prctl() modes.
>>
>> I'm not sure we cover that right now _at all_ and obviously we should
>> describe the new flags.
>>
>> Usama - can you add a patch to this series to do that?
>
> Good point, let's document the interaction with prctl().
>
I have added the following patch for the next revision. I know that a lot
of this will be in the man page as well, but I have gone the way of being very
very explicit of what are all the possible calls that can be made (hopefully thats
the right approach :))
commit 5f290d29741a514d0861d0f99c8b860ba6af9c37
Author: Usama Arif <usamaarif642@gmail.com>
Date: Fri Aug 1 12:05:49 2025 +0100
docs: transhuge: document process level THP controls
This includes the PR_SET_THP_DISABLE/PR_GET_THP_DISABLE pair of
prctl calls as well the newly introduced PR_THP_DISABLE_EXCEPT_ADVISED
flag for the PR_SET_THP_DISABLE prctl call.
Signed-off-by: Usama Arif <usamaarif642@gmail.com>
diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
index 370fba113460..cce0a99beac8 100644
--- a/Documentation/admin-guide/mm/transhuge.rst
+++ b/Documentation/admin-guide/mm/transhuge.rst
@@ -225,6 +225,45 @@ to "always" or "madvise"), and it'll be automatically shutdown when
PMD-sized THP is disabled (when both the per-size anon control and the
top-level control are "never")
+process THP controls
+--------------------
+
+A process can control its own THP behaviour using the ``PR_SET_THP_DISABLE``
+and ``PR_GET_THP_DISABLE`` pair of prctl(2) calls. These calls support the
+following arguments::
+
+ prctl(PR_SET_THP_DISABLE, 1, 0, 0, 0):
+ This will set the MMF_DISABLE_THP_COMPLETELY mm flag which will
+ result in no THPs being faulted in or collapsed, irrespective
+ of global THP controls. This flag and hence the behaviour is
+ inherited across fork(2) and execve(2).
+
+ prctl(PR_SET_THP_DISABLE, 1, PR_THP_DISABLE_EXCEPT_ADVISED, 0, 0):
+ This will set the MMF_DISABLE_THP_EXCEPT_ADVISED mm flag which
+ will result in THPs being faulted in or collapsed only for
+ the following cases:
+ - Global THP controls are set to "always" or "madvise" and
+ the process has madvised the region with either MADV_HUGEPAGE
+ or MADV_COLLAPSE.
+ - Global THP controls is set to "never" and the process has
+ madvised the region with MADV_COLLAPSE.
+ This flag and hence the behaviour is inherited across fork(2)
+ and execve(2).
+
+ prctl(PR_SET_THP_DISABLE, 0, 0, 0, 0):
+ This will clear the MMF_DISABLE_THP_COMPLETELY and
+ MMF_DISABLE_THP_EXCEPT_ADVISED mm flags. The process will
+ behave according to the global THP controls. This behaviour
+ will be inherited across fork(2) and execve(2).
+
+ prctl(PR_GET_THP_DISABLE, 0, 0, 0, 0):
+ This will return the THP disable mm flag status of the process
+ that was set by prctl(PR_SET_THP_DISABLE, ...).
+ i.e.
+ - 1 if MMF_DISABLE_THP_COMPLETELY flag is set
+ - 3 if MMF_DISABLE_THP_EXCEPT_ADVISED flag is set
+ - 0 otherwise.
+
Khugepaged controls
-------------------
>>
>>>
>>> MADV_COLLAPSE is a clear advise that we want to collapse.
>>
>> advise -> advice.
>>
>>>
>>> Note that we still respect the VM_NOHUGEPAGE flag, just like
>>> MADV_COLLAPSE always does. So consequently, MADV_COLLAPSE is now only
>>> refused on VM_NOHUGEPAGE with PR_THP_DISABLE_EXCEPT_ADVISED.
>>
>> You also need to mention the shmem change you've made I think.
>
> Yes.
>
>> >>
>>> Co-developed-by: Usama Arif <usamaarif642@gmail.com>
>>> Signed-off-by: Usama Arif <usamaarif642@gmail.com>
>>> Signed-off-by: David Hildenbrand <david@redhat.com>
>>> ---
>>> include/linux/huge_mm.h | 8 +++++++-
>>> include/uapi/linux/prctl.h | 2 +-
>>> mm/huge_memory.c | 5 +++--
>>> mm/memory.c | 6 ++++--
>>> mm/shmem.c | 2 +-
>>> 5 files changed, 16 insertions(+), 7 deletions(-)
>>>
>>> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
>>> index b0ff54eee81c..aeaf93f8ac2e 100644
>>> --- a/include/linux/huge_mm.h
>>> +++ b/include/linux/huge_mm.h
>>> @@ -329,7 +329,7 @@ struct thpsize {
>>> * through madvise or prctl.
>>> */
>>> static inline bool vma_thp_disabled(struct vm_area_struct *vma,
>>> - vm_flags_t vm_flags)
>>> + vm_flags_t vm_flags, bool forced_collapse)
>>> {
>>> /* Are THPs disabled for this VMA? */
>>> if (vm_flags & VM_NOHUGEPAGE)
>>> @@ -343,6 +343,12 @@ static inline bool vma_thp_disabled(struct vm_area_struct *vma,
>>> */
>>> if (vm_flags & VM_HUGEPAGE)
>>> return false;
>>> + /*
>>> + * Forcing a collapse (e.g., madv_collapse), is a clear advise to
>>
>> advise -> advice.
>>
>>> + * use THPs.
>>> + */
>>> + if (forced_collapse)
>>> + return false;
>>> return test_bit(MMF_DISABLE_THP_EXCEPT_ADVISED, &vma->vm_mm->flags);
>>> }
>>>
>>> diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
>>> index 9c1d6e49b8a9..ee4165738779 100644
>>> --- a/include/uapi/linux/prctl.h
>>> +++ b/include/uapi/linux/prctl.h
>>> @@ -185,7 +185,7 @@ struct prctl_mm_map {
>>> #define PR_SET_THP_DISABLE 41
>>> /*
>>> * Don't disable THPs when explicitly advised (e.g., MADV_HUGEPAGE /
>>> - * VM_HUGEPAGE).
>>> + * VM_HUGEPAGE / MADV_COLLAPSE).
>>
>> This is confusing you're mixing VMA flags with MADV ones... maybe just
>> stick to madvise ones, or add extra context around VM_HUGEPAGE bit?
>
> I don't see anything confusing here, really.
>
> But if it helps you, we can do
> (e.g., MADV_HUGEPAGE / VM_HUGEPAGE, MADV_COLLAPSE).
>
> (reason VM_HUGEPAGE is spelled out is that there might be code where we set VM_HUGEPAGE implicitly in the kernel)
>
>>
>> Would need to be fixed up in a prior commit obviously.
>>
>>> */
>>> # define PR_THP_DISABLE_EXCEPT_ADVISED (1 << 1)
>>> #define PR_GET_THP_DISABLE 42
>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>>> index 85252b468f80..ef5ccb0ec5d5 100644
>>> --- a/mm/huge_memory.c
>>> +++ b/mm/huge_memory.c
>>> @@ -104,7 +104,8 @@ unsigned long __thp_vma_allowable_orders(struct vm_area_struct *vma,
>>> {
>>> const bool smaps = type == TVA_SMAPS;
>>> const bool in_pf = type == TVA_PAGEFAULT;
>>> - const bool enforce_sysfs = type != TVA_FORCED_COLLAPSE;
>>> + const bool forced_collapse = type == TVA_FORCED_COLLAPSE;
>>> + const bool enforce_sysfs = !forced_collapse;
>>
>> Can we just get rid of this enforce_sysfs altogether in patch 2/5 and use
>> forced_collapse?
>
> Let's do that as a separate cleanup on top. I want least churn in that patch.
>
> (had the same idea while writing that patch, but I have other things to focus on than cleaning up all this mess)
>
I am happy to send this cleanup once this series makes it to mm-new and a new revision is not expected.
Thanks!
Usama
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