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

[Vlastimil Babka <vbabka@suse.cz>]

On 9/13/25 03:12, Alexei Starovoitov wrote:
> On Fri, Sep 12, 2025 at 5:36 PM Suren Baghdasaryan <surenb@google.com> wrote:
>>
>> > > > Suren is
>> > > > fixing the condition of VM_BUG_ON_PAGE() in slab_obj_exts(). With this
>> > > > patch, I think, that condition will need to be changed again.
>> > >
>> > > That's orthogonal and I'm not convinced it's correct.
>> > > slab_obj_exts() is doing the right thing. afaict.
>> >
>> > Currently we have
>> >
>> > VM_BUG_ON_PAGE(obj_exts && !(obj_exts & MEMCG_DATA_OBJEXTS))
>> >
>> > but it should be (before your patch) something like:
>> >
>> > VM_BUG_ON_PAGE(obj_exts && !(obj_exts & (MEMCG_DATA_OBJEXTS | OBJEXTS_ALLOC_FAIL)))
>> >
>> > After your patch, hmmm, the previous one would be right again and the
>> > newer one will be the same as the previous due to aliasing. This patch
>> > doesn't need to touch that VM_BUG. Older kernels will need to move to
>> > the second condition though.
>>
>> Correct. Currently slab_obj_exts() will issue a warning when (obj_exts
>> == OBJEXTS_ALLOC_FAIL), which is a perfectly valid state indicating
>> that previous allocation of the vector failed due to memory
>> exhaustion. Changing that warning to:
>>
>> VM_BUG_ON_PAGE(obj_exts && !(obj_exts & (MEMCG_DATA_OBJEXTS |
>> OBJEXTS_ALLOC_FAIL)))
>>
>> will correctly avoid this warning and after your change will still
>> work. (MEMCG_DATA_OBJEXTS | OBJEXTS_ALLOC_FAIL) when
>> (MEMCG_DATA_OBJEXTS == OBJEXTS_ALLOC_FAIL) is technically unnecessary
>> but is good for documenting the conditions we are checking.
> 
> I see what you mean. I feel the comment in slab_obj_exts()
> that explains all that would be better long term than decipher
> from C code. Both are fine, I guess.

I guess perhaps both, having "(MEMCG_DATA_OBJEXTS | OBJEXTS_ALLOC_FAIL)" in
the code (to discover where OBJEXTS_ALLOC_FAIL is important to consider, in
case the flag layout changes again), and a comment explaining what's going on.

Shakeel or Suren, will you sent the fix, including Fixes: ? I can put in
ahead of this series with cc stable in slab/for-next and it shouldn't affect
the series.