Re: [PATCH v4 12/17] mm/huge_memory: move anon_vma handling into the anon split helper

[Zi Yan]

On Mon Sep 7, 2026 at 2:12 PM EDT, Kairui Song via B4 Relay wrote:
> From: Kairui Song <kasong@xxxxxxxxxxx>
>
> Only anon split needs the anon_vma, and it only needs it to unmap and
> remap.  Move the folio_get_anon_vma()/anon_vma_lock_write() pair out of
> __folio_split() into the anon helper next to the folio_mapped()
> check that already gates unmap_folio().
>
> This makes the anon_vma conditional on folio_mapped(), which is a
> behaviour change but should be fine.  folio_get_anon_vma() returns
> NULL whenever !folio_mapped(), so an anon folio with
> folio_mapcount() == 0 used to get -EBUSY from split_huge_page() and is
> now split instead.  The realistic case is a THP that has been fully
> swapped out and is still in the swap cache: swap PTEs do not contribute
> mapcount, so it is !folio_mapped() but still alive.
>
> That should be safe and right to have because:
>
> - folio_ref_freeze() below still rejects a folio that picked up any
>   reference, a mapping or a GUP pin, in the meantime.
>
> - A parallel split is excluded by the folio lock.  The anon_vma
>   write lock was added to serialize split in commit 062f1af2170a
>   ("mm: thp: acquire the anon_vma rwsem for write during split"), when
>   split_huge_page() did not hold the folio lock throughout. commit
>   e9b61f19858a ("thp: reintroduce split_huge_page()") later made the
>   folio lock a caller requirement and added the folio_ref_freeze()
>   scheme, so that has been covered ever since.
>
> - Unmapped path is already exercised by folio_split_unmapped(),
>   and the swap cache split already runs well for a partially
>   swapped-out mapped THP.
>
> - folio_get_anon_vma() and folio_lock_anon_vma_read() both bail out

You mean folio_lock_anon_vma_read() called by others? Since there is no
folio_lock_anon_vma_read() in folio split functions.

>   on !folio_mapped() before taking the anon_vma lock, so there is
>   nothing to lock against.
>
> For mapped folios the anon_vma write lock is now released before
> __folio_split() unlocks the after-split sub-folios, where previously it
> was held across that loop, that window is harmless as the sub-folios
> stay folio-locked and ref pinned.
>
> Signed-off-by: Kairui Song <kasong@xxxxxxxxxxx>
> ---
>  mm/huge_memory.c | 51 ++++++++++++++++++++++++---------------------------
>  1 file changed, 24 insertions(+), 27 deletions(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index c4910c0b6018..53614b875794 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -4005,16 +4005,31 @@ static int __folio_freeze_split_anon(struct folio *folio,
>  	struct swap_cluster_info *ci = NULL;
>  	struct folio *new_folio, *next;
>  	int old_order = folio_order(folio);
> +	struct anon_vma *anon_vma = NULL;
>  	enum ttu_flags ttu_flags = 0;
>  	struct lruvec *lruvec;
> -	bool need_remap = false;
>  	int ret = 0;
>  
> +	/*
> +	 * Unmap/remap needs the anon_vma. The caller does not necessarily
> +	 * hold an mmap_lock that would prevent the anon_vma from
> +	 * disappearing, so we first take a reference and lock it.
> +	 *
> +	 * An unmapped folio needs none of this: folio_get_anon_vma() and

I guess you mean an unmapped folio does not need a ref or a lock on
anon_vma. It is better to be explicit about them.

> +	 * folio_lock_anon_vma_read() both bail out on !folio_mapped()

Mentioning folio_lock_anon_vma_read() is confusing since folio split
does not call it.

> +	 * before taking the lock, and folio_ref_freeze() below still
> +	 * rejects a folio that picked up a reference meanwhile. Note
> +	 * a swapped-out THP counts as unmapped here as swap PTEs do
> +	 * not contribute mapcount, and they are splittable.
> +	 */
>  	if (folio_mapped(folio)) {
> -		need_remap = true;
> +		anon_vma = folio_get_anon_vma(folio);
> +		if (!anon_vma)
> +			return -EBUSY;
> +		anon_vma_lock_write(anon_vma);
>  		ret = unmap_folio(folio);
>  		if (ret)
> -			return ret;
> +			goto out_unlock;
>  	}
>  
>  	local_irq_disable();

Otherwise, LGTM. With the comit message and comments addressed, feel
free to add

Reviewed-by: Zi Yan <ziy@xxxxxxxxxx>


-- 
Best Regards,
Yan, Zi