linux-kernel - Re: [PATCH v2 4/5] mm/slab: Add _

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Date:   Mon, 14 Aug 2023 20:45:30 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        linux-coco@...ts.linux.dev,
        Andrew Morton <akpm@...ux-foundation.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 4/5] mm/slab: Add __free() support for kvfree

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@...ux-foundation.org>
> > > > Cc: Peter Zijlstra <peterz@...radead.org>
> > > > Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> > > > Signed-off-by: Dan Williams <dan.j.williams@...el.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