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Date:   Thu, 9 Jun 2022 16:21:05 +0200
From:   Michal Hocko <mhocko@suse.com>
To:     Christian =?iso-8859-1?Q?K=F6nig?= 
        <ckoenig.leichtzumerken@gmail.com>
Cc:     Christian =?iso-8859-1?Q?K=F6nig?= <christian.koenig@amd.com>,
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        linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
        alexander.deucher@amd.com, daniel@ffwll.ch,
        viro@zeniv.linux.org.uk, akpm@linux-foundation.org,
        hughd@google.com, andrey.grodzovsky@amd.com
Subject: Re: [PATCH 03/13] mm: shmem: provide oom badness for shmem files
Message-ID: <YqIB0bavUeU8Abwl@dhcp22.suse.cz>
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On Thu 09-06-22 16:10:33, Christian K?nig wrote:
> Am 09.06.22 um 14:57 schrieb Michal Hocko:
> > On Thu 09-06-22 14:16:56, Christian K?nig wrote:
> > > Am 09.06.22 um 11:18 schrieb Michal Hocko:
> > > > On Tue 31-05-22 11:59:57, Christian K?nig 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 K?nig <christian.koenig@amd.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?
> 
> Yes, exactly.

So let's say I have a huge sparse shmem file. I will get killed because
the oom_badness of such a file would be large as well...

> > > 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.
> 
> As far as I can see the oom_badness() function is only used in
> oom_kill.c and in procfs to return the oom score. Did I missed
> something?

oom_kill.c implements most of the oom killer functionality. Memcg oom
killing is a part of that. Have a look at select_bad_process.

-- 
Michal Hocko
SUSE Labs