linux-kernel - Re: [PATCH] mm: fix false-positive OVERCOMMIT

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Date:   Wed, 17 Apr 2019 11:05:51 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Roman Gushchin <guro@...com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH] mm: fix false-positive OVERCOMMIT_GUESS failures

On Wed, Apr 17, 2019 at 02:04:17PM +0200, Vlastimil Babka wrote:
> On 4/12/19 10:06 PM, Roman Gushchin wrote:
> > On Fri, Apr 12, 2019 at 03:14:18PM -0400, Johannes Weiner wrote:
> >> With the default overcommit==guess we occasionally run into mmap
> >> rejections despite plenty of memory that would get dropped under
> >> pressure but just isn't accounted reclaimable. One example of this is
> >> dying cgroups pinned by some page cache. A previous case was auxiliary
> >> path name memory associated with dentries; we have since annotated
> >> those allocations to avoid overcommit failures (see d79f7aa496fc ("mm:
> >> treat indirectly reclaimable memory as free in overcommit logic")).
> >>
> >> But trying to classify all allocated memory reliably as reclaimable
> >> and unreclaimable is a bit of a fool's errand. There could be a myriad
> >> of dependencies that constantly change with kernel versions.
> 
> Just wondering, did you find at least one another reclaimable case like
> those path names?

I'm only aware of the cgroup structures which can be pinned by a
dentry, inode, or page cache page. But they're an entire tree of
memory allocations, per-cpu memory regions etc. that would be
impossible to annotate correctly; it's also unreclaimable while the
cgroup is user-visible and only becomes reclaimable once rmdir'd.