linux-kernel - Re: [External] Re: [PATCH] mm/slab: Add a __GFP_ACCOUNT GFP flag check for slab allocation

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Date:   Mon, 15 Jun 2020 21:32:50 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     cl@...ux.com, penberg@...nel.org, rientjes@...gle.com,
        iamjoonsoo.kim@....com, Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Roman Gushchin <guro@...com>
Subject: Re: [External] Re: [PATCH] mm/slab: Add a __GFP_ACCOUNT GFP flag
 check for slab allocation

On Mon, Jun 15, 2020 at 9:08 PM Vlastimil Babka <vbabka@...e.cz> wrote:
>
> On 6/14/20 8:38 AM, Muchun Song wrote:
> > When a kmem_cache is initialized with SLAB_ACCOUNT slab flag, we must
> > not call kmem_cache_alloc with __GFP_ACCOUNT GFP flag. In this case,
> > we can be accounted to kmemcg twice. This is not correct. So we add a
>
> Are you sure? How does that happen?
>
> The only place I see these evaluated is this condition in slab_pre_alloc_hook():
>
>         if (memcg_kmem_enabled() &&
>             ((flags & __GFP_ACCOUNT) || (s->flags & SLAB_ACCOUNT)))
>                 return memcg_kmem_get_cache(s);
>
> And it doesn't matter if one or both are set? Am I missing something?
>
> > __GFP_ACCOUNT GFP flag check for slab allocation.
> >
> > We also introduce a new helper named fixup_gfp_flags to do that check.
> > We can reuse the fixup_gfp_flags for SLAB/SLUB.
> >
> > Signed-off-by: Muchun Song <songmuchun@...edance.com>
> > ---
> >  mm/slab.c | 10 +---------
> >  mm/slab.h | 21 +++++++++++++++++++++
> >  mm/slub.c | 10 +---------
> >  3 files changed, 23 insertions(+), 18 deletions(-)
> >
> > diff --git a/mm/slab.c b/mm/slab.c
> > index 9350062ffc1a..6e0110bef2d6 100644
> > --- a/mm/slab.c
> > +++ b/mm/slab.c
> > @@ -126,8 +126,6 @@
> >
> >  #include <trace/events/kmem.h>
> >
> > -#include     "internal.h"
> > -
> >  #include     "slab.h"
> >
> >  /*
> > @@ -2579,13 +2577,7 @@ static struct page *cache_grow_begin(struct kmem_cache *cachep,
> >        * Be lazy and only check for valid flags here,  keeping it out of the
> >        * critical path in kmem_cache_alloc().
> >        */
> > -     if (unlikely(flags & GFP_SLAB_BUG_MASK)) {
> > -             gfp_t invalid_mask = flags & GFP_SLAB_BUG_MASK;
> > -             flags &= ~GFP_SLAB_BUG_MASK;
> > -             pr_warn("Unexpected gfp: %#x (%pGg). Fixing up to gfp: %#x (%pGg). Fix your code!\n",
> > -                             invalid_mask, &invalid_mask, flags, &flags);
> > -             dump_stack();
> > -     }
> > +     flags = fixup_gfp_flags(cachep, flags);
> >       WARN_ON_ONCE(cachep->ctor && (flags & __GFP_ZERO));
> >       local_flags = flags & (GFP_CONSTRAINT_MASK|GFP_RECLAIM_MASK);
> >
> > diff --git a/mm/slab.h b/mm/slab.h
> > index 815e4e9a94cd..0b91f2a7b033 100644
> > --- a/mm/slab.h
> > +++ b/mm/slab.h
> > @@ -109,6 +109,7 @@ struct memcg_cache_params {
> >  #include <linux/kmemleak.h>
> >  #include <linux/random.h>
> >  #include <linux/sched/mm.h>
> > +#include "internal.h"
> >
> >  /*
> >   * State of the slab allocator.
> > @@ -627,6 +628,26 @@ struct kmem_cache_node {
> >
> >  };
> >
> > +static inline gfp_t fixup_gfp_flags(struct kmem_cache *s, gfp_t flags)
> > +{
> > +     gfp_t invalid_mask = 0;
> > +
> > +     if (unlikely(flags & GFP_SLAB_BUG_MASK))
> > +             invalid_mask |= flags & GFP_SLAB_BUG_MASK;
> > +
> > +     if (unlikely(flags & __GFP_ACCOUNT && s->flags & SLAB_ACCOUNT))
> > +             invalid_mask |= __GFP_ACCOUNT;
> > +
> > +     if (unlikely(invalid_mask)) {
> > +             flags &= ~invalid_mask;
> > +             pr_warn("Unexpected gfp: %#x (%pGg). Fixing up to gfp: %#x (%pGg). Fix your code!\n",
> > +                             invalid_mask, &invalid_mask, flags, &flags);
> > +             dump_stack();
> > +     }
> > +
> > +     return flags;
> > +}
> > +
> >  static inline struct kmem_cache_node *get_node(struct kmem_cache *s, int node)
> >  {
> >       return s->node[node];
> > diff --git a/mm/slub.c b/mm/slub.c
> > index b8f798b50d44..49b5cb7da318 100644
> > --- a/mm/slub.c
> > +++ b/mm/slub.c
> > @@ -37,8 +37,6 @@
> >
> >  #include <trace/events/kmem.h>
> >
> > -#include "internal.h"
> > -
> >  /*
> >   * Lock order:
> >   *   1. slab_mutex (Global Mutex)
> > @@ -1745,13 +1743,7 @@ static struct page *allocate_slab(struct kmem_cache *s, gfp_t flags, int node)
> >
> >  static struct page *new_slab(struct kmem_cache *s, gfp_t flags, int node)
> >  {
> > -     if (unlikely(flags & GFP_SLAB_BUG_MASK)) {
> > -             gfp_t invalid_mask = flags & GFP_SLAB_BUG_MASK;
> > -             flags &= ~GFP_SLAB_BUG_MASK;
> > -             pr_warn("Unexpected gfp: %#x (%pGg). Fixing up to gfp: %#x (%pGg). Fix your code!\n",
> > -                             invalid_mask, &invalid_mask, flags, &flags);
> > -             dump_stack();
> > -     }
> > +     flags = fixup_gfp_flags(s, flags);
> >
> >       return allocate_slab(s,
> >               flags & (GFP_RECLAIM_MASK | GFP_CONSTRAINT_MASK), node);
> >
>

Yeah, you are right. I'm very sorry that I was not thoughtful before.
Please ignore
this patch. Thanks!


-- 
Yours,
Muchun