linux-kernel - Re: [PATCH 03/13] mm: shmem: provide oom badness for shmem files

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Date:   Thu, 9 Jun 2022 14:57:03 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Christian König <christian.koenig@....com>
Cc:     Christian König 
        <ckoenig.leichtzumerken@...il.com>, linux-media@...r.kernel.org,
        linux-kernel@...r.kernel.org, intel-gfx@...ts.freedesktop.org,
        amd-gfx@...ts.freedesktop.org, nouveau@...ts.freedesktop.org,
        linux-tegra@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, alexander.deucher@....com, daniel@...ll.ch,
        viro@...iv.linux.org.uk, akpm@...ux-foundation.org,
        hughd@...gle.com, andrey.grodzovsky@....com
Subject: Re: [PATCH 03/13] mm: shmem: provide oom badness for shmem files

On Thu 09-06-22 14:16:56, Christian König wrote:
> Am 09.06.22 um 11:18 schrieb Michal Hocko:
> > On Tue 31-05-22 11:59:57, Christian König wrote:
> > > This gives the OOM killer an additional hint which processes are
> > > referencing shmem files with potentially no other accounting for them.
> > > 
> > > Signed-off-by: Christian König <christian.koenig@....com>
> > > ---
> > >   mm/shmem.c | 6 ++++++
> > >   1 file changed, 6 insertions(+)
> > > 
> > > diff --git a/mm/shmem.c b/mm/shmem.c
> > > index 4b2fea33158e..a4ad92a16968 100644
> > > --- a/mm/shmem.c
> > > +++ b/mm/shmem.c
> > > @@ -2179,6 +2179,11 @@ unsigned long shmem_get_unmapped_area(struct file *file,
> > >   	return inflated_addr;
> > >   }
> > > +static long shmem_oom_badness(struct file *file)
> > > +{
> > > +	return i_size_read(file_inode(file)) >> PAGE_SHIFT;
> > > +}
> > This doesn't really represent the in memory size of the file, does it?
> 
> Well the file could be partially or fully swapped out as anonymous memory or
> the address space only sparse populated, but even then just using the file
> size as OOM badness sounded like the most straightforward approach to me.

It covers hole as well, right?

> What could happen is that the file is also mmaped and we double account.
> 
> > Also the memcg oom handling could be considerably skewed if the file was
> > shared between more memcgs.
> 
> Yes, and that's one of the reasons why I didn't touched the memcg by this
> and only affected the classic OOM killer.

oom_badness is for all oom handlers, including memcg. Maybe I have
misread an earlier patch but I do not see anything specific to global
oom handling.

-- 
Michal Hocko
SUSE Labs