Re: Mapcount of subpages

[Yang Shi <shy828301@gmail.com>]

On Thu, Sep 23, 2021 at 2:10 PM Hugh Dickins <hughd@google.com> wrote:
>
> On Thu, 23 Sep 2021, Kirill A. Shutemov wrote:
> > On Thu, Sep 23, 2021 at 12:40:14PM +0100, Matthew Wilcox wrote:
> > > On Thu, Sep 23, 2021 at 01:15:16AM -0400, Kent Overstreet wrote:
> > > > On Thu, Sep 23, 2021 at 04:23:12AM +0100, Matthew Wilcox wrote:
> > > > > (compiling that list reminds me that we'll need to sort out mapcount
> > > > > on subpages when it comes time to do this.  ask me if you don't know
> > > > > what i'm talking about here.)
> > > >
> > > > I am curious why we would ever need a mapcount for just part of a page, tell me
> > > > more.
> > >
> > > I would say Kirill is the expert here.  My understanding:
> > >
> > > We have three different approaches to allocating 2MB pages today;
> > > anon THP, shmem THP and hugetlbfs.  Hugetlbfs can only be mapped on a
> > > 2MB boundary, so it has no special handling of mapcount [1].  Anon THP
> > > always starts out as being mapped exclusively on a 2MB boundary, but
> > > then it can be split by, eg, munmap().  If it is, then the mapcount in
> > > the head page is distributed to the subpages.
> >
> > One more complication for anon THP is that it can be shared across fork()
> > and one process may split it while other have it mapped with PMD.
> >
> > > Shmem THP is the tricky one.  You might have a 2MB page in the page cache,
> > > but then have processes which only ever map part of it.  Or you might
> > > have some processes mapping it with a 2MB entry and others mapping part
> > > or all of it with 4kB entries.  And then someone truncates the file to
> > > midway through this page; we split it, and now we need to figure out what
> > > the mapcount should be on each of the subpages.  We handle this by using
> > > ->mapcount on each subpage to record how many non-2MB mappings there are
> > > of that specific page and using ->compound_mapcount to record how many 2MB
> > > mappings there are of the entire 2MB page.  Then, when we split, we just
> > > need to distribute the compound_mapcount to each page to make it correct.
> > > We also have the PageDoubleMap flag to tell us whether anybody has this
> > > 2MB page mapped with 4kB entries, so we can skip all the summing of 4kB
> > > mapcounts if nobody has done that.
> >
> > Possible future complication comes from 1G THP effort. With 1G THP we
> > would have whole hierarchy of mapcounts: 1 PUD mapcount, 512 PMD
> > mapcounts and 262144 PTE mapcounts. (That's one of the reasons I don't
> > think 1G THP is viable.)
> >
> > Note that there are places where exact mapcount accounting is critical:
> > try_to_unmap() may finish prematurely if we underestimate mapcount and
> > overestimating mapcount may lead to superfluous CoW that breaks GUP.
>
> It is critical to know for sure when a page has been completely unmapped:
> but that does not need ptes of subpages to be accounted in the _mapcount
> field of subpages - they just need to be counted in the compound page's
> total_mapcount.
>
> I may be wrong, I never had time to prove it one way or the other: but
> I have a growing suspicion that the *only* reason for maintaining tail
> _mapcounts separately, is to maintain the NR_FILE_MAPPED count exactly
> (in the face of pmd mappings overlapping pte mappings).
>
> NR_FILE_MAPPED being used for /proc/meminfo's "Mapped:" and a couple
> of other such stats files, and for a reclaim heuristic in mm/vmscan.c.
>
> Allow ourselves more slack in NR_FILE_MAPPED accounting (either count
> each pte as if it mapped the whole THP, or don't count a THP's ptes
> at all - you opted for the latter in the "Mlocked:" accounting),
> and I suspect subpage _mapcount could be abandoned.

AFAIK, partial THP unmap may need the _mapcount information of every
subpage otherwise the deferred split can't know what subpages could be
freed.

>
> But you have a different point in mind when you refer to superfluous
> CoW and GUP: I don't know the score there (and I think we are still in
> that halfway zone, since pte CoW was changed to depend on page_count,
> but THP CoW still depending on mapcount).
>
> Hugh
>