Re: [PATCH v2] mm: shmem: ignore sysfs configs for shmem forced collapse

[Lorenzo Stoakes (ARM)]

On Mon, Sep 14, 2026 at 10:12:41AM +0800, Baolin Wang wrote:
> According to Documentation/mm/transhuge.rst, MADV_COLLAPSE is expected to
> ignore any THP or mTHP interface settings.
>
> However, after commit 26c7d8413aaf ("mm: thp: support "THPeligible" semantics
> for mTHP with anonymous shmem"), performing MADV_COLLAPSE on shmem will
> depend on /sys/.../hugepages-2048kB/shmem_enabled being set to "inherit"
> (although that is the default), which can cause MADV_COLLAPSE to fail
> unexpectedly.
>
> Fix this by returning the result of shmem_huge_global_enabled() directly
> when MADV_COLLAPSE is requested, allowing PMD-order collapse while ignoring
> shmem THP/mTHP settings.
>
> Fixes: 26c7d8413aaf ("mm: thp: support "THPeligible" semantics for mTHP with anonymous shmem")
> Reported-by: Kiryl Shutsemau (Meta) <kas@xxxxxxxxxx>
> Closes: https://lore.kernel.org/all/20260908125105.1510704-7-kirill@xxxxxxxxxxxxx/
> Reviewed-by: Kiryl Shutsemau (Meta) <kas@xxxxxxxxxx>
> Acked-by: Zi Yan <ziy@xxxxxxxxxx>
> Reviewed-by: Lance Yang <lance.yang@xxxxxxxxx>
> Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>

Seemes reasonable!

Reviewed-by: Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx>

> ---
> Changes from v1:
>  - Use the correct fixes tag (per Kiryl).
>  - Collect reviewed tags from Kiryl, Zi and Lance. Thanks.
> ---
>  mm/shmem.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/mm/shmem.c b/mm/shmem.c
> index c92ed17dbc4a..b572c60f2af8 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -2049,8 +2049,11 @@ unsigned long shmem_allowable_huge_orders(struct inode *inode,
>
>  	global_orders = shmem_huge_global_enabled(inode, index, write_end,
>  						  shmem_huge_force, vma, vm_flags);
> -	/* Tmpfs huge pages allocation */
> -	if (!vma || !vma_is_anon_shmem(vma))
> +	/*
> +	 * Tmpfs huge pages allocation or forced collapse ignores
> +	 * sysfs configs.
> +	 */
> +	if (!vma || !vma_is_anon_shmem(vma) || shmem_huge_force)
>  		return global_orders;
>
>  	/*
> --
> 2.47.3
>

--
Cheers, Lorenzo