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

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Date:   Thu, 9 Jun 2022 16:10:33 +0200
From:   Christian König <ckoenig.leichtzumerken@...il.com>
To:     Michal Hocko <mhocko@...e.com>,
        Christian König <christian.koenig@....com>
Cc:     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

Am 09.06.22 um 14:57 schrieb Michal Hocko:
> 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?

Yes, exactly.

>
>> 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?

Regards,
Christian.