mm/khugepaged: replace page_mapcount() check by folio_likely_mapped_shared()

We want to limit the use of page_mapcount() to places where absolutely
required, to prepare for kernel configs where we won't keep track of
per-page mapcounts in large folios.

khugepaged is one of the remaining "more challenging" page_mapcount()
users, but we might be able to move away from page_mapcount() without
resulting in a significant behavior change that would warrant
special-casing based on kernel configs.

In 2020, we first added support to khugepaged for collapsing COW-shared
pages via commit 9445689f3b61 ("khugepaged: allow to collapse a page
shared across fork"), followed by support for collapsing PTE-mapped THP in
commit 5503fbf2b0b8 ("khugepaged: allow to collapse PTE-mapped compound
pages") and limiting the memory waste via the "page_count() > 1" check in
commit 71a2c112a0f6 ("khugepaged: introduce 'max_ptes_shared' tunable").

As a default, khugepaged will allow up to half of the PTEs to map shared
pages: where page_mapcount() > 1.  MADV_COLLAPSE ignores the khugepaged
setting.

khugepaged does currently not care about swapcache page references, and
does not check under folio lock: so in some corner cases the "shared vs. 
exclusive" detection might be a bit off, making us detect "exclusive" when
it's actually "shared".

Most of our anonymous folios in the system are usually exclusive.  We
frequently see sharing of anonymous folios for a short period of time,
after which our short-lived suprocesses either quit or exec().

There are some famous examples, though, where child processes exist for a
long time, and where memory is COW-shared with a lot of processes
(webservers, webbrowsers, sshd, ...) and COW-sharing is crucial for
reducing the memory footprint.  We don't want to suddenly change the
behavior to result in a significant increase in memory waste.

Interestingly, khugepaged will only collapse an anonymous THP if at least
one PTE is writable.  After fork(), that means that something (usually a
page fault) populated at least a single exclusive anonymous THP in that
PMD range.

So ...  what happens when we switch to "is this folio mapped shared"
instead of "is this page mapped shared" by using
folio_likely_mapped_shared()?

For "not-COW-shared" folios, small folios and for THPs (large folios) that
are completely mapped into at least one process, switching to
folio_likely_mapped_shared() will not result in a change.

We'll only see a change for COW-shared PTE-mapped THPs that are partially
mapped into all involved processes.

There are two cases to consider:

(A) folio_likely_mapped_shared() returns "false" for a PTE-mapped THP

  If the folio is detected as exclusive, and it actually is exclusive,
  there is no change: page_mapcount() == 1. This is the common case
  without fork() or with short-lived child processes.

  folio_likely_mapped_shared() might currently still detect a folio as
  exclusive although it is shared (false negatives): if the first page is
  not mapped multiple times and if the average per-page mapcount is smaller
  than 1, implying that (1) the folio is partially mapped and (2) if we are
  responsible for many mapcounts by mapping many pages others can't
  ("mostly exclusive") (3) if we are not responsible for many mapcounts by
  mapping little pages ("mostly shared") it won't make a big impact on the
  end result.

  So while we might now detect a page as "exclusive" although it isn't,
  it's not expected to make a big difference in common cases.

(B) folio_likely_mapped_shared() returns "true" for a PTE-mapped THP

  folio_likely_mapped_shared() will never detect a large anonymous folio
  as shared although it is exclusive: there are no false positives.

  If we detect a THP as shared, at least one page of the THP is mapped by
  another process. It could well be that some pages are actually exclusive.
  For example, our child processes could have unmapped/COW'ed some pages
  such that they would now be exclusive to out process, which we now
  would treat as still-shared.

  Examples:
  (1) Parent maps all pages of a THP, child maps some pages. We detect
      all pages in the parent as shared although some are actually
      exclusive.
  (2) Parent maps all but some page of a THP, child maps the remainder.
      We detect all pages of the THP that the parent maps as shared
      although they are all exclusive.

  In (1) we wouldn't collapse a THP right now already: no PTE
  is writable, because a write fault would have resulted in COW of a
  single page and the parent would no longer map all pages of that THP.

  For (2) we would have collapsed a THP in the parent so far, now we
  wouldn't as long as the child process is still alive: unless the child
  process unmaps the remaining THP pages or we decide to split that THP.

  Possibly, the child COW'ed many pages, meaning that it's likely that
  we can populate a THP for our child first, and then for our parent.

  For (2), we are making really bad use of the THP in the first
  place (not even mapped completely in at least one process). If the
  THP would be completely partially mapped, it would be on the deferred
  split queue where we would split it lazily later.

  For short-running child processes, we don't particularly care. For
  long-running processes, the expectation is that such scenarios are
  rather rare: further, a THP might be best placed if most data in the
  PMD range is actually written, implying that we'll have to COW more
  pages first before khugepaged would collapse it.

To summarize, in the common case, this change is not expected to matter
much.  The more common application of khugepaged operates on exclusive
pages, either before fork() or after a child quit.

Can we improve (A)?  Yes, if we implement more precise tracking of "mapped
shared" vs.  "mapped exclusively", we could get rid of the false negatives
completely.

Can we improve (B)?  We could count how many pages of a large folio we map
inside the current page table and detect that we are responsible for most
of the folio mapcount and conclude "as good as exclusive", which might
help in some cases.  ...  but likely, some other mechanism should detect
that the THP is not a good use in the scenario (not even mapped completely
in a single process) and try splitting that folio lazily etc.

We'll move the folio_test_anon() check before our "shared" check, so we
might get more expressive results for SCAN_EXCEED_SHARED_PTE: this order
of checks now matches the one in __collapse_huge_page_isolate().  Extend
documentation.

Link: https://lkml.kernel.org/r/20240424122630.495788-1-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Yang Shi <yang.shi@linux.alibaba.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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