Re: [PATCH 3/3] mm,thp,rmap: lock_compound_mapcounts() on THP mapcounts

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Wed, Nov 02, 2022 at 06:53:45PM -0700, Hugh Dickins wrote:
> Fix the races in maintaining compound_mapcount, subpages_mapcount and
> subpage _mapcount by using PG_locked in the first tail of any compound
> page for a bit_spin_lock() on such modifications; skipping the usual
> atomic operations on those fields in this case.
> 
> Bring page_remove_file_rmap() and page_remove_anon_compound_rmap()
> back into page_remove_rmap() itself.  Rearrange page_add_anon_rmap()
> and page_add_file_rmap() and page_remove_rmap() to follow the same
> "if (compound) {lock} else if (PageCompound) {lock} else {atomic}"
> pattern (with a PageTransHuge in the compound test, like before, to
> avoid BUG_ONs and optimize away that block when THP is not configured).
> Move all the stats updates outside, after the bit_spin_locked section,
> so that it is sure to be a leaf lock.
> 
> Add page_dup_compound_rmap() to manage compound locking versus atomics
> in sync with the rest.  In particular, hugetlb pages are still using
> the atomics: to avoid unnecessary interference there, and because they
> never have subpage mappings; but this exception can easily be changed.
> Conveniently, page_dup_compound_rmap() turns out to suit an anon THP's
> __split_huge_pmd_locked() too.
> 
> bit_spin_lock() is not popular with PREEMPT_RT folks: but PREEMPT_RT
> sensibly excludes TRANSPARENT_HUGEPAGE already, so its only exposure
> is to the non-hugetlb non-THP pte-mapped compound pages (with large
> folios being currently dependent on TRANSPARENT_HUGEPAGE).  There is
> never any scan of subpages in this case; but we have chosen to use
> PageCompound tests rather than PageTransCompound tests to gate the
> use of lock_compound_mapcounts(), so that page_mapped() is correct on
> all compound pages, whether or not TRANSPARENT_HUGEPAGE is enabled:
> could that be a problem for PREEMPT_RT, when there is contention on
> the lock - under heavy concurrent forking for example?  If so, then it
> can be turned into a sleeping lock (like folio_lock()) when PREEMPT_RT.
> 
> A simple 100 X munmap(mmap(2GB, MAP_SHARED|MAP_POPULATE, tmpfs), 2GB)
> took 18 seconds on small pages, and used to take 1 second on huge pages,
> but now takes 115 milliseconds on huge pages.  Mapping by pmds a second
> time used to take 860ms and now takes 86ms; mapping by pmds after mapping
> by ptes (when the scan is needed) used to take 870ms and now takes 495ms.
> Mapping huge pages by ptes is largely unaffected but variable: between 5%
> faster and 5% slower in what I've recorded.  Contention on the lock is
> likely to behave worse than contention on the atomics behaved.
> 
> Signed-off-by: Hugh Dickins <hughd@google.com>

Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>

-- 
  Kiryl Shutsemau / Kirill A. Shutemov