Re: [RFC] memcg: Convert mc_target.page to mc_target.folio

[Michal Hocko <mhocko@suse.com>]

On Fri 18-03-22 03:49:27, Matthew Wilcox wrote:
> On Thu, Mar 17, 2022 at 12:58:46PM +0100, Michal Hocko wrote:
> > On Wed 16-03-22 20:31:30, Matthew Wilcox wrote:
> > > This is a fairly mechanical change to convert mc_target.page to
> > > mc_target.folio.  This is a prerequisite for converting
> > > find_get_incore_page() to find_get_incore_folio().  But I'm not
> > > convinced it's right, and I'm not convinced the existing code is
> > > quite right either.
> > > 
> > > In particular, the code in hunk @@ -6036,28 +6041,26 @@ needs
> > > careful review.  There are also assumptions in here that a memory
> > > allocation is never larger than a PMD, which is true today, but I've
> > > been asked about larger allocations.
> > 
> > Could you be more specific about those usecases? Are they really
> > interested in supporting larger pages for the memcg migration which is
> > v1 only feature? Or you are interested merely to have the code more
> > generic?
> 
> Ah!  I didn't realise memcg migration was a v1-only feature.  I think
> that makes all of the questions much less interesting.  I've done some
> more reading, and it seems like all of this is "best effort", so it
> doesn't really matter if some folios get skipped.

Yes.
[...]

> That makes sense.  I think the case that's currently mishandled is a
> THP in tmpfs which is misaligned when mapped to userspace.  It's
> skipped, even if the entire THP is mapped.  But maybe that simply
> doesn't matter.
> 
> I suppose the question is: Do we care if mappings of files are not
> migrated to the new memcg?  I'm getting a sense that the answer is "no",
> and if we actually ended up skipping all file mappings, it wouldn't
> matter.

Yes, I wouldn't lose sleep over that. You are not introducing a new
regression. The feature is mostly deprecated (along with the whole v1)
so we tend to prefer bug-to-bug compatibility rather than making the
code more complex to solve a theoretical problem (or at least a problem
that nobody is complaining about).

Thanks!
-- 
Michal Hocko
SUSE Labs