Re: [PATCH] mm/slub: check pfmemalloc_match in slab_alloc_node fastpath

[Ohoon Kwon]

Oh I was not aware of those histories.
I checked the commits, and it seems it is better to leave it
optimized(as it is now).

Thanks for your help.
Ohhoon Kwon.



On Sun, Apr 10, 2022 at 5:12 PM Hyeonggon Yoo <42.hyeyoo@xxxxxxxxx> wrote:
>
> On Sun, Apr 10, 2022 at 01:02:23AM +0900, Ohhoon Kwon wrote:
> > If current alloc context does not have __GFP_MEMALLOC in its gfpflags,
> > then slab objects that were previously created with __GFP_MEMALLOC
> > should not be given.
> >
> > This criteria is well kept in slab alloc slowpath:
> > When gfpflags does not contain __GFP_MEMALLOC but if per-cpu slab page
> > was allocated with __GFP_MEMALLOC, then allocator first deactivates
> > per-cpu slab page and then again allocates new slab page with the
> > current context's gfpflags.
> >
> > However, this criteria is not checked in fastpath.
> > It should also be checked in the fastpath, too.
> >
> > Signed-off-by: Ohhoon Kwon <ohkwon1043@xxxxxxxxx>
> > ---
> >  mm/slub.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/mm/slub.c b/mm/slub.c
> > index 74d92aa4a3a2..c77cd548e106 100644
> > --- a/mm/slub.c
> > +++ b/mm/slub.c
> > @@ -3179,7 +3179,8 @@ static __always_inline void *slab_alloc_node(struct kmem_cache *s, struct list_l
> >        * there is a suitable cpu freelist.
> >        */
> >       if (IS_ENABLED(CONFIG_PREEMPT_RT) ||
> > -         unlikely(!object || !slab || !node_match(slab, node))) {
> > +         unlikely(!object || !slab || !node_match(slab, node) ||
> > +                     !pfmemalloc_match(slab, gfpflags))) {
> >               object = __slab_alloc(s, gfpflags, node, addr, c);
> >       } else {
> >               void *next_object = get_freepointer_safe(s, object);
>
> The missing pfmemalloc check in fastpath was intended.
>
> pfmemalloc check in fast did exist in Mel's commit 072bb0aa5e0629 ("mm:
> sl[au]b: add knowledge of PFMEMALLOC reserve pages").
>
> But later removed by Christoph's commit 5091b74a95d4 ("mm: slub: optimise
> the SLUB fast path to avoid pfmemalloc checks").
>
> Any thoughts?
>
> Thanks!
>
> --
> Thanks,
> Hyeonggon