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[PATCH v2 4/5] mm/slab: Add __free() support for kvfree

Dan Williams posted 5 patches 2 years, 4 months ago
[PATCH v2 4/5] mm/slab: Add __free() support for kvfree
Posted by Dan Williams 2 years, 4 months ago 
Allow for the declaration of variables that trigger kvfree() when they
go out of scope.

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
---
 include/linux/slab.h |    2 ++
 1 file changed, 2 insertions(+)

diff --git a/include/linux/slab.h b/include/linux/slab.h
index 848c7c82ad5a..241025367943 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -746,6 +746,8 @@ static inline __alloc_size(1, 2) void *kvcalloc(size_t n, size_t size, gfp_t fla
 extern void *kvrealloc(const void *p, size_t oldsize, size_t newsize, gfp_t flags)
 		      __realloc_size(3);
 extern void kvfree(const void *addr);
+DEFINE_FREE(kvfree, void *, if (_T) kvfree(_T))
+
 extern void kvfree_sensitive(const void *addr, size_t len);
 
 unsigned int kmem_cache_size(struct kmem_cache *s);
Re: [PATCH v2 4/5] mm/slab: Add __free() support for kvfree
Posted by Greg Kroah-Hartman 2 years, 4 months ago 
On Mon, Aug 14, 2023 at 12:43:32AM -0700, Dan Williams wrote:
> Allow for the declaration of variables that trigger kvfree() when they
> go out of scope.
> 
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Signed-off-by: Dan Williams <dan.j.williams@intel.com>
> ---
>  include/linux/slab.h |    2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/include/linux/slab.h b/include/linux/slab.h
> index 848c7c82ad5a..241025367943 100644
> --- a/include/linux/slab.h
> +++ b/include/linux/slab.h
> @@ -746,6 +746,8 @@ static inline __alloc_size(1, 2) void *kvcalloc(size_t n, size_t size, gfp_t fla
>  extern void *kvrealloc(const void *p, size_t oldsize, size_t newsize, gfp_t flags)
>  		      __realloc_size(3);
>  extern void kvfree(const void *addr);
> +DEFINE_FREE(kvfree, void *, if (_T) kvfree(_T))

No need to check _T before calling this, right (as was also pointed out
earlier).

thanks,

greg k-h
Re: [PATCH v2 4/5] mm/slab: Add __free() support for kvfree
Posted by Peter Zijlstra 2 years, 4 months ago 
On Mon, Aug 14, 2023 at 05:31:27PM +0200, Greg Kroah-Hartman wrote:
> On Mon, Aug 14, 2023 at 12:43:32AM -0700, Dan Williams wrote:
> > Allow for the declaration of variables that trigger kvfree() when they
> > go out of scope.
> > 
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> > Signed-off-by: Dan Williams <dan.j.williams@intel.com>
> > ---
> >  include/linux/slab.h |    2 ++
> >  1 file changed, 2 insertions(+)
> > 
> > diff --git a/include/linux/slab.h b/include/linux/slab.h
> > index 848c7c82ad5a..241025367943 100644
> > --- a/include/linux/slab.h
> > +++ b/include/linux/slab.h
> > @@ -746,6 +746,8 @@ static inline __alloc_size(1, 2) void *kvcalloc(size_t n, size_t size, gfp_t fla
> >  extern void *kvrealloc(const void *p, size_t oldsize, size_t newsize, gfp_t flags)
> >  		      __realloc_size(3);
> >  extern void kvfree(const void *addr);
> > +DEFINE_FREE(kvfree, void *, if (_T) kvfree(_T))
> 
> No need to check _T before calling this, right (as was also pointed out
> earlier).

Well, that does mean you get an unconditional call to kvfree() in the
success case. Linus argued against this.

This way the compiler sees:

	buf = NULL;
	if (buf)
		kvfree(buf);

and goes: 'let me clean that up for you'. And all is well.
Re: [PATCH v2 4/5] mm/slab: Add __free() support for kvfree
Posted by Lukas Wunner 1 year, 11 months ago 
On Mon, Aug 14, 2023 at 06:17:31PM +0200, Peter Zijlstra wrote:
> On Mon, Aug 14, 2023 at 05:31:27PM +0200, Greg Kroah-Hartman wrote:
> > On Mon, Aug 14, 2023 at 12:43:32AM -0700, Dan Williams wrote:
> > > Allow for the declaration of variables that trigger kvfree() when they
> > > go out of scope.
> > > 
> > > Cc: Andrew Morton <akpm@linux-foundation.org>
> > > Cc: Peter Zijlstra <peterz@infradead.org>
> > > Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> > > Signed-off-by: Dan Williams <dan.j.williams@intel.com>
> > > ---
> > >  include/linux/slab.h |    2 ++
> > >  1 file changed, 2 insertions(+)
> > > 
> > > diff --git a/include/linux/slab.h b/include/linux/slab.h
> > > index 848c7c82ad5a..241025367943 100644
> > > --- a/include/linux/slab.h
> > > +++ b/include/linux/slab.h
> > > @@ -746,6 +746,8 @@ static inline __alloc_size(1, 2) void *kvcalloc(size_t n, size_t size, gfp_t fla
> > >  extern void *kvrealloc(const void *p, size_t oldsize, size_t newsize, gfp_t flags)
> > >  		      __realloc_size(3);
> > >  extern void kvfree(const void *addr);
> > > +DEFINE_FREE(kvfree, void *, if (_T) kvfree(_T))
> > 
> > No need to check _T before calling this, right (as was also pointed out
> > earlier).
> 
> Well, that does mean you get an unconditional call to kvfree() in the
> success case. Linus argued against this.
> 
> This way the compiler sees:
> 
> 	buf = NULL;
> 	if (buf)
> 		kvfree(buf);
> 
> and goes: 'let me clean that up for you'. And all is well.

Have you actually verified that assumption in the generated Assembler code?

The kernel is compiled with -fno-delete-null-pointer-checks since commit
a3ca86aea507 ("Add '-fno-delete-null-pointer-checks' to gcc CFLAGS").

So NULL pointer checks are *not* optimized away even if the compiler
knows that a pointer is NULL.

Background story:
https://lwn.net/Articles/342330/

Thanks,

Lukas
Re: [PATCH v2 4/5] mm/slab: Add __free() support for kvfree
Posted by Dan Williams 1 year, 11 months ago 
Lukas Wunner wrote:
> On Mon, Aug 14, 2023 at 06:17:31PM +0200, Peter Zijlstra wrote:
> > On Mon, Aug 14, 2023 at 05:31:27PM +0200, Greg Kroah-Hartman wrote:
> > > On Mon, Aug 14, 2023 at 12:43:32AM -0700, Dan Williams wrote:
> > > > Allow for the declaration of variables that trigger kvfree() when they
> > > > go out of scope.
> > > > 
> > > > Cc: Andrew Morton <akpm@linux-foundation.org>
> > > > Cc: Peter Zijlstra <peterz@infradead.org>
> > > > Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> > > > Signed-off-by: Dan Williams <dan.j.williams@intel.com>
> > > > ---
> > > >  include/linux/slab.h |    2 ++
> > > >  1 file changed, 2 insertions(+)
> > > > 
> > > > diff --git a/include/linux/slab.h b/include/linux/slab.h
> > > > index 848c7c82ad5a..241025367943 100644
> > > > --- a/include/linux/slab.h
> > > > +++ b/include/linux/slab.h
> > > > @@ -746,6 +746,8 @@ static inline __alloc_size(1, 2) void *kvcalloc(size_t n, size_t size, gfp_t fla
> > > >  extern void *kvrealloc(const void *p, size_t oldsize, size_t newsize, gfp_t flags)
> > > >  		      __realloc_size(3);
> > > >  extern void kvfree(const void *addr);
> > > > +DEFINE_FREE(kvfree, void *, if (_T) kvfree(_T))
> > > 
> > > No need to check _T before calling this, right (as was also pointed out
> > > earlier).
> > 
> > Well, that does mean you get an unconditional call to kvfree() in the
> > success case. Linus argued against this.
> > 
> > This way the compiler sees:
> > 
> > 	buf = NULL;
> > 	if (buf)
> > 		kvfree(buf);
> > 
> > and goes: 'let me clean that up for you'. And all is well.
> 
> Have you actually verified that assumption in the generated Assembler code?
> 
> The kernel is compiled with -fno-delete-null-pointer-checks since commit
> a3ca86aea507 ("Add '-fno-delete-null-pointer-checks' to gcc CFLAGS").
> 
> So NULL pointer checks are *not* optimized away even if the compiler
> knows that a pointer is NULL.

Interesting, I am not sure how -fno-delete-null-pointer-checks plays
into this, but I can confirm that Peter's expectations are being met in
a routine with:

DEFINE_FREE(pci_dev_put, struct pci_dev *, if (_T) pci_dev_put(_T))

...without that conditional the assembly is:

   0xffffffff819ad129 <+41>:	call   0xffffffff81800840 <pci_get_domain_bus_and_slot>
   0xffffffff819ad12e <+46>:	mov    %rax,%r12
   0xffffffff819ad131 <+49>:	test   %rax,%rax
   0xffffffff819ad134 <+52>:	je     0xffffffff819ad154 <cxl_cper_event_call+84>
   0xffffffff819ad136 <+54>:	mov    %rax,%rdi
   0xffffffff819ad139 <+57>:	call   0xffffffff817f5f10 <pci_dev_lock>
   0xffffffff819ad13e <+62>:	cmpq   $0xffffffff82c681c0,0x80(%r12)
   0xffffffff819ad14a <+74>:	je     0xffffffff819ad160 <cxl_cper_event_call+96>
   0xffffffff819ad14c <+76>:	mov    %r12,%rdi
   0xffffffff819ad14f <+79>:	call   0xffffffff817f5fa0 <pci_dev_unlock>
   0xffffffff819ad154 <+84>:	pop    %rbx
   0xffffffff819ad155 <+85>:	mov    %r12,%rdi
   0xffffffff819ad158 <+88>:	pop    %rbp
   0xffffffff819ad159 <+89>:	pop    %r12
   0xffffffff819ad15b <+91>:	jmp    0xffffffff817fe1e0 <pci_dev_put>

...i.e. the check for NULL at 0xffffffff819ad134 jumps to do an
unnecessary pci_dev_put(). With the conditional in the macro the
sequence is:

   0xffffffff819ad129 <+41>:	call   0xffffffff81800840 <pci_get_domain_bus_and_slot>
   0xffffffff819ad12e <+46>:	test   %rax,%rax
   0xffffffff819ad131 <+49>:	je     0xffffffff819ad18c <cxl_cper_event_call+140>
   0xffffffff819ad133 <+51>:	mov    %rax,%r12
   0xffffffff819ad136 <+54>:	mov    %rax,%rdi
   0xffffffff819ad139 <+57>:	call   0xffffffff817f5f10 <pci_dev_lock>
   0xffffffff819ad13e <+62>:	cmpq   $0xffffffff82c681c0,0x80(%r12)
   0xffffffff819ad14a <+74>:	je     0xffffffff819ad160 <cxl_cper_event_call+96>
   0xffffffff819ad14c <+76>:	mov    %r12,%rdi
   0xffffffff819ad14f <+79>:	call   0xffffffff817f5fa0 <pci_dev_unlock>
   ...
   0xffffffff819ad18c <+140>:	pop    %rbx
   0xffffffff819ad18d <+141>:	pop    %rbp
   0xffffffff819ad18e <+142>:	pop    %r12
   0xffffffff819ad190 <+144>:	jmp    0xffffffff81efc6a0 <__x86_return_thunk>

...i.e. optimize away the pci_dev_put() and return directly when @pdev
is already known to be NULL. So empirically
-fno-delete-null-pointer-checks still allows for redundant NULL checks
to be optimized.
Re: [PATCH v2 4/5] mm/slab: Add __free() support for kvfree
Posted by Greg Kroah-Hartman 2 years, 4 months ago 
On Mon, Aug 14, 2023 at 06:17:31PM +0200, Peter Zijlstra wrote:
> On Mon, Aug 14, 2023 at 05:31:27PM +0200, Greg Kroah-Hartman wrote:
> > On Mon, Aug 14, 2023 at 12:43:32AM -0700, Dan Williams wrote:
> > > Allow for the declaration of variables that trigger kvfree() when they
> > > go out of scope.
> > > 
> > > Cc: Andrew Morton <akpm@linux-foundation.org>
> > > Cc: Peter Zijlstra <peterz@infradead.org>
> > > Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> > > Signed-off-by: Dan Williams <dan.j.williams@intel.com>
> > > ---
> > >  include/linux/slab.h |    2 ++
> > >  1 file changed, 2 insertions(+)
> > > 
> > > diff --git a/include/linux/slab.h b/include/linux/slab.h
> > > index 848c7c82ad5a..241025367943 100644
> > > --- a/include/linux/slab.h
> > > +++ b/include/linux/slab.h
> > > @@ -746,6 +746,8 @@ static inline __alloc_size(1, 2) void *kvcalloc(size_t n, size_t size, gfp_t fla
> > >  extern void *kvrealloc(const void *p, size_t oldsize, size_t newsize, gfp_t flags)
> > >  		      __realloc_size(3);
> > >  extern void kvfree(const void *addr);
> > > +DEFINE_FREE(kvfree, void *, if (_T) kvfree(_T))
> > 
> > No need to check _T before calling this, right (as was also pointed out
> > earlier).
> 
> Well, that does mean you get an unconditional call to kvfree() in the
> success case. Linus argued against this.
> 
> This way the compiler sees:
> 
> 	buf = NULL;
> 	if (buf)
> 		kvfree(buf);
> 
> and goes: 'let me clean that up for you'. And all is well.

Ah, didn't realize that, ok, nevermind :)
Re: [PATCH v2 4/5] mm/slab: Add __free() support for kvfree
Posted by Greg Kroah-Hartman 2 years, 4 months ago 
On Mon, Aug 14, 2023 at 08:44:43PM +0200, Greg Kroah-Hartman wrote:
> On Mon, Aug 14, 2023 at 06:17:31PM +0200, Peter Zijlstra wrote:
> > On Mon, Aug 14, 2023 at 05:31:27PM +0200, Greg Kroah-Hartman wrote:
> > > On Mon, Aug 14, 2023 at 12:43:32AM -0700, Dan Williams wrote:
> > > > Allow for the declaration of variables that trigger kvfree() when they
> > > > go out of scope.
> > > > 
> > > > Cc: Andrew Morton <akpm@linux-foundation.org>
> > > > Cc: Peter Zijlstra <peterz@infradead.org>
> > > > Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> > > > Signed-off-by: Dan Williams <dan.j.williams@intel.com>
> > > > ---
> > > >  include/linux/slab.h |    2 ++
> > > >  1 file changed, 2 insertions(+)
> > > > 
> > > > diff --git a/include/linux/slab.h b/include/linux/slab.h
> > > > index 848c7c82ad5a..241025367943 100644
> > > > --- a/include/linux/slab.h
> > > > +++ b/include/linux/slab.h
> > > > @@ -746,6 +746,8 @@ static inline __alloc_size(1, 2) void *kvcalloc(size_t n, size_t size, gfp_t fla
> > > >  extern void *kvrealloc(const void *p, size_t oldsize, size_t newsize, gfp_t flags)
> > > >  		      __realloc_size(3);
> > > >  extern void kvfree(const void *addr);
> > > > +DEFINE_FREE(kvfree, void *, if (_T) kvfree(_T))
> > > 
> > > No need to check _T before calling this, right (as was also pointed out
> > > earlier).
> > 
> > Well, that does mean you get an unconditional call to kvfree() in the
> > success case. Linus argued against this.
> > 
> > This way the compiler sees:
> > 
> > 	buf = NULL;
> > 	if (buf)
> > 		kvfree(buf);
> > 
> > and goes: 'let me clean that up for you'. And all is well.
> 
> Ah, didn't realize that, ok, nevermind :)

Note, a comment should be added because in a year or so someone is going
to come along and try to "clean this up" and we will have forgotten why
it's a bad idea to do so.

thanks,

greg k-h

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