On 9 Jun 2026, at 14:38, David Hildenbrand (Arm) wrote: > On 6/9/26 20:10, Andrew Morton wrote: >> On Tue, 9 Jun 2026 06:12:49 -0400 "Michael S. Tsirkin" <[email protected]> >> wrote: >> >>> TestSetPageHWPoison() is called without zone->lock, so its atomic >>> update to page->flags can race with non-atomic flag operations >>> that run under zone->lock in the buddy allocator. >>> >>> In particular, __free_pages_prepare() does: >>> >>> page->flags.f &= ~PAGE_FLAGS_CHECK_AT_PREP; >>> >>> This non-atomic read-modify-write, while correctly excluding >>> __PG_HWPOISON from the mask, can still lose a concurrent >>> TestSetPageHWPoison if the read happens before the poison bit >>> is set and the write happens after. Will only get worse if/when >>> we add more non-atomic flag operations. >>> >>> Fix by acquiring zone->lock around TestSetPageHWPoison and >>> around ClearPageHWPoison in the retry path. This >>> serializes with all buddy flag manipulation. The cost is >>> negligible: one lock/unlock in an extremely rare path >>> (hardware memory errors). >>> >>> Note: SetPageHWPoison and TestClearPageHWPoison calls elsewhere >>> in this file operate on pages already removed from the buddy >>> allocator or on non-buddy pages (DAX, hugetlb), so they do not >>> need zone->lock protection. >> >> Sashiko is saying this doesn't do anything "Because >> __free_pages_prepare() executes entirely locklessly". Did it goof? >> >> https://sashiko.dev/#/patchset/df06b66fe4ff8e925ee0714955abc2183a727b90.1780998980.git....@redhat.com > > Battle of the bots: it's right.
Yep, __free_pages_prepare() changes the page flag without holding zone->lock. Best Regards, Yan, Zi
