Re: [PATCH mm-unstable v1] mm: add a total mapcount for large folios

[David Hildenbrand]

On 09.08.23 21:17, David Hildenbrand wrote:
> On 09.08.23 21:07, Ryan Roberts wrote:
> > On 09/08/2023 09:32, David Hildenbrand wrote:
> > > Let's track the total mapcount for all large folios in the first subpage.
> > >
> > > The total mapcount is what we actually want to know in folio_mapcount()
> > > and it is also sufficient for implementing folio_mapped(). This also
> > > gets rid of any "raceiness" concerns as expressed in
> > > folio_total_mapcount().
> > >
> > > With sub-PMD THP becoming more important and things looking promising
> > > that we will soon get support for such anon THP, we want to avoid looping
> > > over all pages of a folio just to calculate the total mapcount. Further,
> > > we may soon want to use the total mapcount in other context more
> > > frequently, so prepare for reading it efficiently and atomically.
> > >
> > > Make room for the total mapcount in page[1] of the folio by moving
> > > _nr_pages_mapped to page[2] of the folio: it is not applicable to hugetlb
> > > -- and with the total mapcount in place probably also not desirable even
> > > if PMD-mappable hugetlb pages could get PTE-mapped at some point -- so we
> > > can overlay the hugetlb fields.
> > >
> > > Note that we currently don't expect any order-1 compound pages / THP in
> > > rmap code, and that such support is not planned. If ever desired, we could
> > > move the compound mapcount to another page, because it only applies to
> > > PMD-mappable folios that are definitely larger. Let's avoid consuming
> > > more space elsewhere for now -- we might need more space for a different
> > > purpose in some subpages soon.
> > >
> > > Maintain the total mapcount also for hugetlb pages. Use the total mapcount
> > > to implement folio_mapcount(), total_mapcount(), folio_mapped() and
> > > page_mapped().
> > >
> > > We can now get rid of folio_total_mapcount() and
> > > folio_large_is_mapped(), by just inlining reading of the total mapcount.
> > >
> > > _nr_pages_mapped is now only used in rmap code, so not accidentially
> > > externally where it might be used on arbitrary order-1 pages. The remaining
> > > usage is:
> > >
> > > (1) Detect how to adjust stats: NR_ANON_MAPPED and NR_FILE_MAPPED
> > >    -> If we would account the total folio as mapped when mapping a
> > >       page (based on the total mapcount), we could remove that usage.
> > >
> > > (2) Detect when to add a folio to the deferred split queue
> > >    -> If we would apply a different heuristic, or scan using the rmap on
> > >       the memory reclaim path for partially mapped anon folios to
> > >       split them, we could remove that usage as well.
> > >
> > > So maybe, we can simplify things in the future and remove
> > > _nr_pages_mapped. For now, leave these things as they are, they need more
> > > thought. Hugh really did a nice job implementing that precise tracking
> > > after all.
> > >
> > > Note: Not adding order-1 sanity checks to the file_rmap functions for
> > >         now. For anon pages, they are certainly not required right now.
> > >
> > > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > > Cc: Jonathan Corbet <corbet@xxxxxxx>
> > > Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> > > Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> > > Cc: "Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx>
> > > Cc: Ryan Roberts <ryan.roberts@xxxxxxx>
> > > Cc: Yin Fengwei <fengwei.yin@xxxxxxxxx>
> > > Cc: Yang Shi <shy828301@xxxxxxxxx>
> > > Cc: Zi Yan <ziy@xxxxxxxxxx>
> > > Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> >
> > Other than the nits and query on zeroing _total_mapcount below, LGTM. If zeroing
> > is correct:
> >
> > Reviewed-by: Ryan Roberts <ryan.roberts@xxxxxxx>
>
> Thanks for the review!
>
> [...]
>
> > >    
> > >    static inline int total_mapcount(struct page *page)
> >
> > nit: couldn't total_mapcount() just be implemented as a wrapper around
> > folio_mapcount()? What's the benefit of PageCompound() check instead of just
> > getting the folio and checking if it's large? i.e:
>
> Good point, let me take a look tomorrow if the compiler can optimize in
> both cases equally well.

I checked by adjusting total_mapcount():

Before:

        if (PageTransHuge(page) && total_mapcount(page) > 1)
ffffffff81411931:       4c 89 e7                mov    %r12,%rdi
ffffffff81411934:       e8 f7 b1 ff ff          call   ffffffff8140cb30 <PageTransHuge>
ffffffff81411939:       85 c0                   test   %eax,%eax
ffffffff8141193b:       74 29                   je     ffffffff81411966 <migrate_misplaced_page+0x166>
ffffffff8141193d:       49 8b 04 24             mov    (%r12),%rax
        return test_bit(PG_head, &page->flags) ||
ffffffff81411941:       a9 00 00 01 00          test   $0x10000,%eax
ffffffff81411946:       0f 85 1f 01 00 00       jne    ffffffff81411a6b <migrate_misplaced_page+0x26b>
               READ_ONCE(page->compound_head) & 1;
ffffffff8141194c:       49 8b 44 24 08          mov    0x8(%r12),%rax
        return test_bit(PG_head, &page->flags) ||
ffffffff81411951:       a8 01                   test   $0x1,%al
ffffffff81411953:       0f 85 12 01 00 00       jne    ffffffff81411a6b <migrate_misplaced_page+0x26b>
ffffffff81411959:       41 8b 44 24 30          mov    0x30(%r12),%eax
                return atomic_read(&page->_mapcount) + 1;
ffffffff8141195e:       83 c0 01                add    $0x1,%eax
ffffffff81411961:       83 f8 01                cmp    $0x1,%eax
ffffffff81411964:       7f 77                   jg     ffffffff814119dd <migrate_misplaced_page+0x1dd>

So a total of 10 instructions after handling the mov/call/test/je for PageTransHuge().

After:

        if (PageTransHuge(page) && total_mapcount(page) > 1)
ffffffff81411931:       4c 89 e7                mov    %r12,%rdi
ffffffff81411934:       e8 f7 b1 ff ff          call   ffffffff8140cb30 <PageTransHuge>
ffffffff81411939:       85 c0                   test   %eax,%eax
ffffffff8141193b:       74 2f                   je     ffffffff8141196c <migrate_misplaced_page+0x16c>
        unsigned long head = READ_ONCE(page->compound_head);
ffffffff8141193d:       49 8b 44 24 08          mov    0x8(%r12),%rax
        if (unlikely(head & 1))
ffffffff81411942:       a8 01                   test   $0x1,%al
ffffffff81411944:       0f 85 fc 05 00 00       jne    ffffffff81411f46 <migrate_misplaced_page+0x746>
ffffffff8141194a:       0f 1f 44 00 00          nopl   0x0(%rax,%rax,1)
                return page;
ffffffff8141194f:       4c 89 e0                mov    %r12,%rax
ffffffff81411952:       48 8b 10                mov    (%rax),%rdx
        if (likely(!folio_test_large(folio)))
ffffffff81411955:       f7 c2 00 00 01 00       test   $0x10000,%edx
ffffffff8141195b:       0f 85 da 05 00 00       jne    ffffffff81411f3b <migrate_misplaced_page+0x73b>
ffffffff81411961:       8b 40 30                mov    0x30(%rax),%eax
                return atomic_read(&folio->_mapcount) + 1;
ffffffff81411964:       83 c0 01                add    $0x1,%eax
ffffffff81411967:       83 f8 01                cmp    $0x1,%eax
ffffffff8141196a:       7f 77                   jg     ffffffff814119e3 <migrate_misplaced_page+0x1e3>

So a total of 11 (+ 1 NOP) instructions after handling the mov/call/test/je for PageTransHuge().

Essentially one more MOV instruction.

I guess nobody really cares :)

--
Cheers,

David / dhildenb