linux-kernel - Re: [PATCH mm v3] memcg: enable memory accounting in __alloc_pages

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Message-ID: <CALvZod6K6UXxDrkHp=mVDV7O-fAtmRkgMDngPazBhcyDUNxy_Q@mail.gmail.com>
Date:   Wed, 13 Oct 2021 09:41:15 -0700
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Vasily Averin <vvs@...tuozzo.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Roman Gushchin <guro@...com>,
        Uladzislau Rezki <urezki@...il.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Cgroups <cgroups@...r.kernel.org>, Linux MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>, kernel@...nvz.org
Subject: Re: [PATCH mm v3] memcg: enable memory accounting in __alloc_pages_bulk

On Tue, Oct 12, 2021 at 11:24 AM Michal Hocko <mhocko@...e.com> wrote:
>
> On Tue 12-10-21 09:08:38, Shakeel Butt wrote:
> > On Tue, Oct 12, 2021 at 8:36 AM Michal Hocko <mhocko@...e.com> wrote:
> > >
> > > On Tue 12-10-21 17:58:21, Vasily Averin wrote:
> > > > Enable memory accounting for bulk page allocator.
> > >
> > > ENOCHANGELOG
> > >
> > > And I have to say I am not very happy about the solution. It adds a very
> > > tricky code where it splits different charging steps apart.
> > >
> > > Would it be just too inefficient to charge page-by-page once all pages
> > > are already taken away from the pcp lists? This bulk should be small so
> > > this shouldn't really cause massive problems. I mean something like
> > >
> > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > > index b37435c274cf..8bcd69195ef5 100644
> > > --- a/mm/page_alloc.c
> > > +++ b/mm/page_alloc.c
> > > @@ -5308,6 +5308,10 @@ unsigned long __alloc_pages_bulk(gfp_t gfp, int preferred_nid,
> > >
> > >         local_unlock_irqrestore(&pagesets.lock, flags);
> > >
> > > +       if (memcg_kmem_enabled() && (gfp & __GFP_ACCOUNT)) {
> > > +               /* charge pages here */
> > > +       }
> >
> > It is not that simple because __alloc_pages_bulk only allocate pages
> > for empty slots in the page_array provided by the caller.
> >
> > The failure handling for post charging would be more complicated.
>
> If this is really that complicated (I haven't tried) then it would be
> much more simple to completely skip the bulk allocator for __GFP_ACCOUNT
> rather than add a tricky code. The bulk allocator is meant to be used
> for ultra hot paths and memcg charging along with the reclaim doesn't
> really fit into that model anyway. Or are there any actual users who
> really need bulk allocator optimization and also need memcg accounting?

Bulk allocator is being used for vmalloc and we have several
kvmalloc() with __GFP_ACCOUNT allocations.

It seems like Vasily has some ideas, so let's wait for his next version.