Re: [PATCH slab v5 5/6] slab: Reuse first bit for OBJEXTS_ALLOC_FAIL

[Shakeel Butt <shakeel.butt@linux.dev>]

On Fri, Sep 12, 2025 at 02:24:26PM -0700, Alexei Starovoitov wrote:
> On Fri, Sep 12, 2025 at 2:03 PM Suren Baghdasaryan <surenb@google.com> wrote:
> >
> > On Fri, Sep 12, 2025 at 12:27 PM Shakeel Butt <shakeel.butt@linux.dev> wrote:
> > >
> > > +Suren, Roman
> > >
> > > On Mon, Sep 08, 2025 at 06:00:06PM -0700, Alexei Starovoitov wrote:
> > > > From: Alexei Starovoitov <ast@kernel.org>
> > > >
> > > > Since the combination of valid upper bits in slab->obj_exts with
> > > > OBJEXTS_ALLOC_FAIL bit can never happen,
> > > > use OBJEXTS_ALLOC_FAIL == (1ull << 0) as a magic sentinel
> > > > instead of (1ull << 2) to free up bit 2.
> > > >
> > > > Signed-off-by: Alexei Starovoitov <ast@kernel.org>
> > >
> > > Are we low on bits that we need to do this or is this good to have
> > > optimization but not required?
> >
> > That's a good question. After this change MEMCG_DATA_OBJEXTS and
> > OBJEXTS_ALLOC_FAIL will have the same value and they are used with the
> > same field (page->memcg_data and slab->obj_exts are aliases). Even if
> > page_memcg_data_flags can never be used for slab pages I think
> > overlapping these bits is not a good idea and creates additional
> > risks. Unless there is a good reason to do this I would advise against
> > it.
> 
> Completely disagree. You both missed the long discussion
> during v4. The other alternative was to increase alignment
> and waste memory. Saving the bit is obviously cleaner.
> The next patch is using the saved bit.

I will check out that discussion and it would be good to summarize that
in the commit message.