linux-kernel - Re: [PATCH] memcg: page_alloc: skip bulk allocator for __GFP

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Date:   Thu, 14 Oct 2021 01:08:36 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     Roman Gushchin <guro@...com>, Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Uladzislau Rezki <urezki@...il.com>,
        Vasily Averin <vvs@...tuozzo.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Cgroups <cgroups@...r.kernel.org>, Linux MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] memcg: page_alloc: skip bulk allocator for __GFP_ACCOUNT

On Wed, Oct 13, 2021 at 04:45:35PM -0700, Shakeel Butt wrote:
> On Wed, Oct 13, 2021 at 4:15 PM Roman Gushchin <guro@...com> wrote:
> >
> [...]
> > > >
> > > > Isn't it a bit too aggressive?
> > > >
> > > > How about
> > > >     if (WARN_ON_ONCE(gfp & __GFP_ACCOUNT))
> > >
> > > We actually know that kvmalloc(__GFP_ACCOUNT) users exist and can
> > > trigger bulk page allocator through vmalloc, so I don't think the
> > > warning would be any helpful.
> > >
> > > >        gfp &= ~__GFP_ACCOUNT;
> > >
> > > Bulk allocator is best effort, so callers have adequate fallbacks.
> > > Transparently disabling accounting would be unexpected.
> >
> > I see...
> >
> > Shouldn't we then move this check to an upper level?
> >
> > E.g.:
> >
> > if (!(gfp & __GFP_ACCOUNT))
> >    call_into_bulk_allocator();
> > else
> >    call_into_per_page_allocator();
> >
> 
> If we add this check in the upper level (e.g. in vm_area_alloc_pages()
> ) then I think we would need WARN_ON_ONCE(gfp & __GFP_ACCOUNT) in the
> bulk allocator to detect future users.
> 
> At the moment I am more inclined towards this patch's approach. Let's
> say in future we find there is a __GFP_ACCOUNT allocation which can
> benefit from bulk allocator and we decide to add such support in bulk
> allocator then we would not need to change the bulk allocator callers
> at that time just the bulk allocator.

I agree with you.  Let's apply the patch as-is.