linux-kernel - Re: [PATCH v2] mm/slub: Simplify __kmem_cache

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Date:   Wed, 15 Jun 2022 19:09:38 +0800
From:   Xiongwei Song <sxwjean@...il.com>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Hyeonggon Yoo <42.hyeyoo@...il.com>,
        Xiongwei Song <sxwjean@...com>,
        Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Muchun Song <songmuchun@...edance.com>,
        "linux-mm @ kvack . org" <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Xiongwei Song <xiongwei.song@...driver.com>
Subject: Re: [PATCH v2] mm/slub: Simplify __kmem_cache_alias()

On Wed, Jun 15, 2022 at 4:37 PM Vlastimil Babka <vbabka@...e.cz> wrote:
>
> On 6/5/22 09:04, Xiongwei Song wrote:
> > On Sat, Jun 4, 2022 at 5:43 PM Hyeonggon Yoo <42.hyeyoo@...il.com> wrote:
> >>
> >> On Fri, Jun 03, 2022 at 10:35:55PM +0800, sxwjean@...com wrote:
> >> > From: Xiongwei Song <xiongwei.song@...driver.com>
> >> >
> >> > There is no need to do anything if sysfs_slab_alias() return nonzero
> >> > value after getting a mergeable cache.
> >> >
> >> > Signed-off-by: Xiongwei Song <xiongwei.song@...driver.com>
> >> > Reviewed-by: Muchun Song <songmuchun@...edance.com>
> >> > ---
> >> > v2: Collect Reviewed-by tag from Muchun.
>
> Hmm I added v1 (with the Reviewed tag) before getting to the v2 thread. But
> I think it's fine, see below.
>
> >> > ---
> >> >  mm/slub.c | 8 +++-----
> >> >  1 file changed, 3 insertions(+), 5 deletions(-)
> >> >
> >> > diff --git a/mm/slub.c b/mm/slub.c
> >> > index d8d5abf49f5f..9444277d669a 100644
> >> > --- a/mm/slub.c
> >> > +++ b/mm/slub.c
> >> > @@ -4861,6 +4861,9 @@ __kmem_cache_alias(const char *name, unsigned int size, unsigned int align,
> >> >
> >> >       s = find_mergeable(size, align, flags, name, ctor);
> >> >       if (s) {
> >> > +             if (sysfs_slab_alias(s, name))
> >> > +                     return NULL;
> >> > +
> >> >               s->refcount++;
> >> >
> >>
> >> I think we should not expose sysfs attributes before initializing
> >> what can be read via sysfs attribute (object_size).
>
> Hmm I don't think they are unitialized. They have an old value from the
> cache we are merging with, which is updated if the new aliased cache has a
> larger one.
> So yeah we might briefly during creation expose an alias that will have an
> incorrect value, but I doubt anything will break. The values are not stable
> anyway as new aliases are added, as we are bumping them for the 'root' cache
> and all aliases that share it already.
>
> >> >               /*
> >> > @@ -4869,11 +4872,6 @@ __kmem_cache_alias(const char *name, unsigned int size, unsigned int align,
> >> >                */
> >> >               s->object_size = max(s->object_size, size);
> >>
> >> this calculation should be done before sysfs_slab_alias().
> >
> > Yeah, understood. Should we restore s->object_size and s->inuse if
> > sysfs_slab_alias() returns non zero value?
>
> And by bailing out early this patch effectively achieves that, so I'd say
> it's a better state than before the patch so I'll keep it unless proven
> otherwise. Thanks!

Thank you for your comments Vlastimil and Hyeonggon.

Regards,
Xiongwei

>
> > Regards,
> > Xiongwwei
> >
> >>
> >> Thanks,
> >> Hyeonggon
> >>
> >> >               s->inuse = max(s->inuse, ALIGN(size, sizeof(void *)));
> >> > -
> >> > -             if (sysfs_slab_alias(s, name)) {
> >> > -                     s->refcount--;
> >> > -                     s = NULL;
> >> > -             }
> >> >       }
> >> >
> >> >       return s;
> >> > --
> >> > 2.30.2
> >> >
> >>
>