Re: [PATCH v2] mm/huge_memory: prevent losing PG_waiters in __split_folio_to_order()

[David Hildenbrand]

On Fri, 06 Nov 2026 13:28:46 -0500, Zi Yan wrote:
> __split_folio_to_order() calls prep_compound_page() on each new head page
> to create a compound page after clear_compound_head(). prep_compound_page()
> uses non atomic __SetPageHead(). It is mostly OK, since the folio is locked
> and frozen and no one else is supposed to modify the folio's flags.
> 
> However, code calling migration_entry_wait() can reach the folio by PFN,
> then access its flags. It is because softleaf_entry_wait_on_locked() used
> by migration_entry_wait() sets PG_waiters concurrently without taking a
> reference and the frozen refcount during split cannot block it. Meanwhile,
> __SetPageHead() does a plain read and write, so PG_waiters atomically set
> by softleaf_entry_wait_on_locked() will be lost if the set happens after
> __SetPageHead()'s read and before its write.
> 
> [...]

Applied, thanks!

tree: mm/core.git
branch: for-next-fixes

Applied patches will appear soon in mm-next, followed by linux-next.

Please report any outstanding bugs that were missed during review, so we
can either drop the patch series or decide how to fix it up.

It is encouraged to provide Acked-bys, Reviewed-bys, and Tested-bys even
though the patches have already been applied. If possible, patch trailers
will be updated.

Note that commit hashes shown below are subject to change due to rebase,
trailer updates or similar. Further, this is not a guarantee that the
patches will go upstream. If in doubt, please check the listed branch.

[1/1] mm/huge_memory: prevent losing PG_waiters in __split_folio_to_order()
      https://git.kernel.org/mm/core/c/654fdaf4

-- 
Cheers,

David