Re: [Patch v2] mm/huge_memory: fix early failure try_to_migrate() when split huge pmd for shared thp

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

On Wed,  4 Feb 2026 00:42:19 +0000 Wei Yang <richard.weiyang@gmail.com> wrote:

> Commit 60fbb14396d5 ("mm/huge_memory: adjust try_to_migrate_one() and
> split_huge_pmd_locked()") return false unconditionally after
> split_huge_pmd_locked() which may fail early during try_to_migrate() for
> shared thp. This will lead to unexpected folio split failure.
> 
> One way to reproduce:
> 
>     Create an anonymous thp range and fork 512 children, so we have a
>     thp shared mapped in 513 processes. Then trigger folio split with
>     /sys/kernel/debug/split_huge_pages debugfs to split the thp folio to
>     order 0.
> 
> Without the above commit, we can successfully split to order 0.
> With the above commit, the folio is still a large folio.
> 
> The reason is the above commit return false after split pmd
> unconditionally in the first process and break try_to_migrate().
> 
> The tricky thing in above reproduce method is current debugfs interface
> leverage function split_huge_pages_pid(), which will iterate the whole
> pmd range and do folio split on each base page address. This means it
> will try 512 times, and each time split one pmd from pmd mapped to pte
> mapped thp. If there are less than 512 shared mapped process,
> the folio is still split successfully at last. But in real world, we
> usually try it for once.
> 
> This patch fixes this by restart page_vma_mapped_walk() after
> split_huge_pmd_locked(). Because split_huge_pmd_locked() may fall back to
> (freeze = false) if folio_try_share_anon_rmap_pmd() fails and the PMD is
> just split instead of split to migration entry. Restart
> page_vma_mapped_walk() and let try_to_migrate_one() try on each PTE
> again and fail try_to_migrate() early if it fails.
> 
> Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
> Fixes: 60fbb14396d5 ("mm/huge_memory: adjust try_to_migrate_one() and split_huge_pmd_locked()")

Cool, thanks.

> Cc: Gavin Guo <gavinguo@igalia.com>
> Cc: "David Hildenbrand (Red Hat)" <david@kernel.org>
> Cc: Zi Yan <ziy@nvidia.com>
> Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
> Cc: Lance Yang <lance.yang@linux.dev>
> Cc: <stable@vger.kernel.org>

Why cc:stable?  In other words, what is the userspace-visible runtime
effect of this bug?