linux-kernel - Re: [PATCH 1/1] mm/slab: fix 'variable obj

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Date: Tue, 2 Jul 2024 18:58:05 -0700
From: Suren Baghdasaryan <surenb@...gle.com>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: akpm@...ux-foundation.org, kent.overstreet@...ux.dev, 
	pasha.tatashin@...een.com, souravpanda@...gle.com, keescook@...omium.org, 
	linux-mm@...ck.org, linux-kernel@...r.kernel.org, 
	kernel test robot <lkp@...el.com>
Subject: Re: [PATCH 1/1] mm/slab: fix 'variable obj_exts set but not used' warning

On Tue, Jul 2, 2024 at 8:16 AM Suren Baghdasaryan <surenb@...gle.com> wrote:
>
> On Tue, Jul 2, 2024 at 9:31 AM Vlastimil Babka <vbabka@...e.cz> wrote:
> >
> > On 6/30/24 9:20 PM, Suren Baghdasaryan wrote:
> > > On Mon, Jun 17, 2024 at 3:04 AM Vlastimil Babka <vbabka@...e.cz> wrote:
> > >>
> > >> On 6/15/24 12:59 AM, Suren Baghdasaryan wrote:
> > >> > slab_post_alloc_hook() uses prepare_slab_obj_exts_hook() to obtain
> > >> > slabobj_ext object. Currently the only user of slabobj_ext object in
> > >> > this path is memory allocation profiling, therefore when it's not enabled
> > >> > this object is not needed. This also generates a warning when compiling
> > >> > with CONFIG_MEM_ALLOC_PROFILING=n. Move the code under this configuration
> > >> > to fix the warning. If more slabobj_ext users appear in the future, the
> > >> > code will have to be changed back to call prepare_slab_obj_exts_hook().
> > >> >
> > >> > Fixes: 4b8736964640 ("mm/slab: add allocation accounting into slab allocation and free paths")
> > >> > Reported-by: kernel test robot <lkp@...el.com>
> > >> > Closes: https://lore.kernel.org/oe-kbuild-all/202406150444.F6neSaiy-lkp@intel.com/
> > >> > Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
> > >>
> > >> Acked-by: Vlastimil Babka <vbabka@...e.cz>
> > >>
> > >> But it seems to me we could remove the whole #ifdef if current->alloc_tag
> > >> (which doesn't exist with !MEM_ALLOC_PROFILING) had an access helper, or
> > >> there was a alloc_tag_add_current() variant?
> > >
> > > Hmm. I'll check if current->alloc_tag is the only reason for this
> > > ifdef. If so then you are correct and we can simplify this code.
> >
> > The fix is now in mm-hotfixes-stable but we can cleanup for the future as a
> > non-hotfix.
>
> Yes, it's on my TODO list now. Thanks!

obj_exts->ref was also undefined when !MEM_ALLOC_PROFILING, so I moved
that call into a separate hook. It's posted at
https://lore.kernel.org/all/20240703015354.3370503-1-surenb@google.com/
Thanks,
Suren.

>
> >