6287b7dae8
Commit 4b51634cd16a, introducing the COMPOUND_MAPPED bit, paid attention to the impossibility of subpages_mapcount ever appearing negative; but did not attend to those races in which it can momentarily appear larger than thought possible. These arise from how page_remove_rmap() first decrements page->_mapcount or compound_mapcount, then, if that transition goes negative (logical 0), decrements subpages_mapcount. The initial decrement lets a racing page_add_*_rmap() reincrement _mapcount or compound_mapcount immediately, and then in rare cases its corresponding increment of subpages_mapcount may be completed before page_remove_rmap()'s decrement. There could even (with increasing unlikelihood) be a series of increments intermixed with the decrements. In practice, checking subpages_mapcount with a temporary WARN on range, has caught values of 0x1000000 (2*COMPOUND_MAPPED, when move_pages() was using remove_migration_pmd()) and 0x800201 (do_huge_pmd_wp_page() using __split_huge_pmd()): page_add_anon_rmap() racing page_remove_rmap(), as predicted. I certainly found it harder to reason about than when bit_spin_locked, but the easy case gives a clue to how to handle the harder case. The easy case being the three !(nr & COMPOUND_MAPPED) checks, which should obviously be replaced by (nr < COMPOUND_MAPPED) checks - to count a page as compound mapped, even while the bit in that position is 0. The harder case is when trying to decide how many subpages are newly covered or uncovered, when compound map is first added or last removed: not knowing all that racily happened between first and second atomic ops. But the easy way to handle that, is again to count the page as compound mapped all the while that its subpages_mapcount indicates so - ignoring the _mapcount or compound_mapcount transition while it is on the way to being reversed. Link: https://lkml.kernel.org/r/4388158-3092-a960-ff2d-55f2b0fe4ef8@google.com Fixes: 4b51634cd16a ("mm,thp,rmap: subpages_mapcount COMPOUND_MAPPED if PMD-mapped") Signed-off-by: Hugh Dickins <hughd@google.com> Cc: David Hildenbrand <david@redhat.com> Cc: James Houghton <jthoughton@google.com> Cc: Johannes Weiner <hannes@cmpxchg.org> Cc: John Hubbard <jhubbard@nvidia.com> Cc: "Kirill A . Shutemov" <kirill@shutemov.name> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Matthew Wilcox <willy@infradead.org> Cc: Miaohe Lin <linmiaohe@huawei.com> Cc: Mike Kravetz <mike.kravetz@oracle.com> Cc: Mina Almasry <almasrymina@google.com> Cc: Muchun Song <songmuchun@bytedance.com> Cc: Naoya Horiguchi <naoya.horiguchi@linux.dev> Cc: Peter Xu <peterx@redhat.com> Cc: Sidhartha Kumar <sidhartha.kumar@oracle.com> Cc: Vlastimil Babka <vbabka@suse.cz> Cc: Yang Shi <shy828301@gmail.com> Cc: Zach O'Keefe <zokeefe@google.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.