Re: [PATCH v3] mm/huge_memory: fix folio isn't locked in softleaf_to_folio()

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 19 Mar 2026 09:25:41 +0800 Jinjiang Tu <tujinjiang@huawei.com> wrote:

> On arm64 server, we found folio that get from migration entry isn't locked
> in softleaf_to_folio(). This issue triggers when mTHP splitting and
> zap_nonpresent_ptes() races, and the root cause is lack of memory barrier
> in softleaf_to_folio(). The race is as follows:
> 
> 	CPU0                                             CPU1
> 
> deferred_split_scan()                              zap_nonpresent_ptes()
>   lock folio
>   split_folio()
>     unmap_folio()
>       change ptes to migration entries
>     __split_folio_to_order()                         softleaf_to_folio()
>       set flags(including PG_locked) for tail pages    folio = pfn_folio(softleaf_to_pfn(entry))
>       smp_wmb()                                        VM_WARN_ON_ONCE(!folio_test_locked(folio))
>       prep_compound_page() for tail pages
> 
> In __split_folio_to_order(), smp_wmb() guarantees page flags of tail pages
> are visible before the tail page becomes non-compound. smp_wmb() should
> be paired with smp_rmb() in softleaf_to_folio(), which is missed. As a
> result, if zap_nonpresent_ptes() accesses migration entry that stores
> tail pfn, softleaf_to_folio() may see the updated compound_head of tail
> page before page->flags.

Please describe the userspace-visible runtime effects of this bug.

> To fix it, add missing smp_rmb() if the softleaf entry is migration entry
> in softleaf_to_folio() and softleaf_to_page().
> 
> Fixes: e9b61f19858a ("thp: reintroduce split_huge_page()")

So we know whether a -stable backport is needed.  Thanks.