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

[Lukas Wunner]

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@xxxxxxxxxxxxxxxxxxxx>
> > > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > > Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> > > Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
> > > ---
> > >  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