Re: [PATCH splitout] mm: memory-failure: serialize TestSetPageHWPoison with zone->lock

[Andrew Morton]

On Tue, 9 Jun 2026 06:12:49 -0400 "Michael S. Tsirkin" <mst@xxxxxxxxxx> 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.mst@xxxxxxxxxx