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

[Christian König]

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@xxxxxxx>
> > > > ---
> > > >    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.