linux-kernel - Re: [PATCH 0/5] blkcg: Limit maximum number of aio requests available for cgroup

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Date:   Thu, 7 Dec 2017 14:44:47 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     Benjamin LaHaise <ben@...munityfibre.ca>
Cc:     Kirill Tkhai <ktkhai@...tuozzo.com>, Tejun Heo <tj@...nel.org>,
        axboe@...nel.dk, viro@...iv.linux.org.uk,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-aio@...ck.org
Subject: Re: [PATCH 0/5] blkcg: Limit maximum number of aio requests
 available for cgroup

On 12/06, Benjamin LaHaise wrote:
>
> On Wed, Dec 06, 2017 at 06:32:56PM +0100, Oleg Nesterov wrote:
> >
> > No. Again, this memory is not properly accounted, and unlike mlock()ed
> > memory it is visible to shrinker which will do the unnecessary work on
> > memory shortage which in turn will lead to unnecessary page faults.
> >
> > So let me repeat, shouldn't we at least do mapping_set_unevictable() in
> > aio_private_file() ?

... and probably account this memory in ->pinned_vm

> Send a patch then!

I have no idea how to test this change, and personally I don't reallly care
about aio,

> I don't know why you're asking rather than sending a
> patch to do this if you think it is needed.

Because you are maintainer, and I naively thought it is always fine to
ask the maintainer if you think the code is not correct or sub-optimal.
Sorry for bothering you.

> > > > triggers OOM-killer which kills sshd and other daemons on my machine.
> > > > These pages were not even faulted in (or the shrinker can unmap them),
> > > > the kernel can not know who should be blamed.
> > >
> > > The OOM-killer killed the wrong process: News at 11.
> >
> > Well. I do not think we should blame OOM-killer in this case. But as I
> > said this is not a bug-report or something like this, I agree this is
> > a minor issue.
>
> I do think the OOM-killer is doing the wrong thing here.  If process X is
> the only one that is allocating gobs of memory,

aio_setup_ring() does find_or_create_page(file->f_mapping), this adds
the page to page cache. Again, this memory looks _reclaimable_ but it
is not because ctx->ring_pages has a reference.

I do not understand how we can blame OOM-killer, it should not kill the
task which blows the page cache, and this is how io_setup() looks to vm.

Quite possibly I missed something, please correct me.

Oleg.