From 4b51634cd16a01b2be0f6b69cc0dae63de4751f2 Mon Sep 17 00:00:00 2001 From: Hugh Dickins Date: Tue, 22 Nov 2022 01:49:36 -0800 Subject: mm,thp,rmap: subpages_mapcount COMPOUND_MAPPED if PMD-mapped Can the lock_compound_mapcount() bit_spin_lock apparatus be removed now? Yes. Not by atomic64_t or cmpxchg games, those get difficult on 32-bit; but if we slightly abuse subpages_mapcount by additionally demanding that one bit be set there when the compound page is PMD-mapped, then a cascade of two atomic ops is able to maintain the stats without bit_spin_lock. This is harder to reason about than when bit_spin_locked, but I believe safe; and no drift in stats detected when testing. When there are racing removes and adds, of course the sequence of operations is less well- defined; but each operation on subpages_mapcount is atomically good. What might be disastrous, is if subpages_mapcount could ever fleetingly appear negative: but the pte lock (or pmd lock) these rmap functions are called under, ensures that a last remove cannot race ahead of a first add. Continue to make an exception for hugetlb (PageHuge) pages, though that exception can be easily removed by a further commit if necessary: leave subpages_mapcount 0, don't bother with COMPOUND_MAPPED in its case, just carry on checking compound_mapcount too in folio_mapped(), page_mapped(). Evidence is that this way goes slightly faster than the previous implementation in all cases (pmds after ptes now taking around 103ms); and relieves us of worrying about contention on the bit_spin_lock. Link: https://lkml.kernel.org/r/3978f3ca-5473-55a7-4e14-efea5968d892@google.com Signed-off-by: Hugh Dickins Acked-by: Kirill A. Shutemov Cc: Dan Carpenter Cc: David Hildenbrand Cc: James Houghton Cc: Johannes Weiner Cc: John Hubbard Cc: Linus Torvalds Cc: Matthew Wilcox Cc: Miaohe Lin Cc: Mike Kravetz Cc: Mina Almasry Cc: Muchun Song Cc: Naoya Horiguchi Cc: Peter Xu Cc: Sidhartha Kumar Cc: Vlastimil Babka Cc: Yang Shi Cc: Yu Zhao Cc: Zach O'Keefe Signed-off-by: Andrew Morton --- Documentation/mm/transhuge.rst | 7 +++---- 1 file changed, 3 insertions(+), 4 deletions(-) (limited to 'Documentation/mm') diff --git a/Documentation/mm/transhuge.rst b/Documentation/mm/transhuge.rst index af4c9d70321d..ec3dc5b04226 100644 --- a/Documentation/mm/transhuge.rst +++ b/Documentation/mm/transhuge.rst @@ -118,15 +118,14 @@ pages: succeeds on tail pages. - map/unmap of PMD entry for the whole compound page increment/decrement - ->compound_mapcount, stored in the first tail page of the compound page. + ->compound_mapcount, stored in the first tail page of the compound page; + and also increment/decrement ->subpages_mapcount (also in the first tail) + by COMPOUND_MAPPED when compound_mapcount goes from -1 to 0 or 0 to -1. - map/unmap of sub-pages with PTE entry increment/decrement ->_mapcount on relevant sub-page of the compound page, and also increment/decrement ->subpages_mapcount, stored in first tail page of the compound page, when _mapcount goes from -1 to 0 or 0 to -1: counting sub-pages mapped by PTE. - In order to have race-free accounting of sub-pages mapped, changes to - sub-page ->_mapcount, ->subpages_mapcount and ->compound_mapcount are - are all locked by bit_spin_lock of PG_locked in the first tail ->flags. split_huge_page internally has to distribute the refcounts in the head page to the tail pages before clearing all PG_head/tail bits from the page -- cgit