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Message-ID: <e545a153-bca2-47fd-a768-dc310977de15@nvidia.com>
Date: Wed, 26 Nov 2025 15:14:15 +1100
From: Balbir Singh <balbirs@...dia.com>
To: Zi Yan <ziy@...dia.com>, David Hildenbrand <david@...nel.org>,
Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Baolin Wang <baolin.wang@...ux.alibaba.com>,
"Liam R. Howlett" <Liam.Howlett@...cle.com>, Nico Pache <npache@...hat.com>,
Ryan Roberts <ryan.roberts@....com>, Dev Jain <dev.jain@....com>,
Barry Song <baohua@...nel.org>, Lance Yang <lance.yang@...ux.dev>,
Miaohe Lin <linmiaohe@...wei.com>, Naoya Horiguchi
<nao.horiguchi@...il.com>, Wei Yang <richard.weiyang@...il.com>,
linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/4] mm/huge_memory: change folio_split_supported() to
folio_check_splittable()
On 11/26/25 14:50, Zi Yan wrote:
> folio_split_supported() used in try_folio_split_to_order() requires
> folio->mapping to be non NULL, but current try_folio_split_to_order() does
> not check it. There is no issue in the current code, since
> try_folio_split_to_order() is only used in truncate_inode_partial_folio(),
> where folio->mapping is not NULL.
>
> To prevent future misuse, move folio->mapping NULL check (i.e., folio is
> truncated) into folio_split_supported(). Since folio->mapping NULL check
> returns -EBUSY and folio_split_supported() == false means -EINVAL, change
> folio_split_supported() return type from bool to int and return error
> numbers accordingly. Rename folio_split_supported() to
> folio_check_splittable() to match the return type change.
>
> While at it, move is_huge_zero_folio() check and folio_test_writeback()
> check into folio_check_splittable() and add kernel-doc.
>
> Remove all warnings inside folio_check_splittable() and give warnings
> in __folio_split() instead, so that bool warns parameter can be removed.
>
> Signed-off-by: Zi Yan <ziy@...dia.com>
> Reviewed-by: Wei Yang <richard.weiyang@...il.com>
> ---
> include/linux/huge_mm.h | 6 ++--
> mm/huge_memory.c | 76 +++++++++++++++++++++++------------------
> 2 files changed, 46 insertions(+), 36 deletions(-)
>
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index 1d439de1ca2c..66105a90b4c3 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -375,8 +375,8 @@ int __split_huge_page_to_list_to_order(struct page *page, struct list_head *list
> int folio_split_unmapped(struct folio *folio, unsigned int new_order);
> int min_order_for_split(struct folio *folio);
> int split_folio_to_list(struct folio *folio, struct list_head *list);
> -bool folio_split_supported(struct folio *folio, unsigned int new_order,
> - enum split_type split_type, bool warns);
> +int folio_check_splittable(struct folio *folio, unsigned int new_order,
> + enum split_type split_type);
> int folio_split(struct folio *folio, unsigned int new_order, struct page *page,
> struct list_head *list);
>
> @@ -407,7 +407,7 @@ static inline int split_huge_page_to_order(struct page *page, unsigned int new_o
> static inline int try_folio_split_to_order(struct folio *folio,
> struct page *page, unsigned int new_order)
> {
> - if (!folio_split_supported(folio, new_order, SPLIT_TYPE_NON_UNIFORM, /* warns= */ false))
> + if (folio_check_splittable(folio, new_order, SPLIT_TYPE_NON_UNIFORM))
> return split_huge_page_to_order(&folio->page, new_order);
> return folio_split(folio, new_order, page, NULL);
> }
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 041b554c7115..771df0c02a4a 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -3688,15 +3688,40 @@ static int __split_unmapped_folio(struct folio *folio, int new_order,
> return 0;
> }
>
> -bool folio_split_supported(struct folio *folio, unsigned int new_order,
> - enum split_type split_type, bool warns)
> +/**
> + * folio_check_splittable() - check if a folio can be split to a given order
> + * @folio: folio to be split
> + * @new_order: the smallest order of the after split folios (since buddy
> + * allocator like split generates folios with orders from @folio's
> + * order - 1 to new_order).
> + * @split_type: uniform or non-uniform split
> + *
> + * folio_check_splittable() checks if @folio can be split to @new_order using
> + * @split_type method. The truncated folio check must come first.
> + *
> + * Context: folio must be locked.
> + *
> + * Return: 0 - @folio can be split to @new_order, otherwise an error number is
> + * returned.
> + */
> +int folio_check_splittable(struct folio *folio, unsigned int new_order,
> + enum split_type split_type)
> {
> + VM_WARN_ON_FOLIO(!folio_test_locked(folio), folio);
> + /*
> + * Folios that just got truncated cannot get split. Signal to the
> + * caller that there was a race.
> + *
> + * TODO: this will also currently refuse folios without a mapping in the
> + * swapcache (shmem or to-be-anon folios).
> + */
> + if (!folio_test_anon(folio) && !folio->mapping)
> + return -EBUSY;
> +
Nit: Shouldn't the order of check be
if (!folio->mapping && !folio_test_anon(folio))
works better if folio->mapping is NULL
> if (folio_test_anon(folio)) {
> /* order-1 is not supported for anonymous THP. */
> - VM_WARN_ONCE(warns && new_order == 1,
> - "Cannot split to order-1 folio");
> if (new_order == 1)
> - return false;
> + return -EINVAL;
> } else if (split_type == SPLIT_TYPE_NON_UNIFORM || new_order) {
> if (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) &&
> !mapping_large_folio_support(folio->mapping)) {
> @@ -3717,9 +3742,7 @@ bool folio_split_supported(struct folio *folio, unsigned int new_order,
> * case, the mapping does not actually support large
> * folios properly.
> */
> - VM_WARN_ONCE(warns,
> - "Cannot split file folio to non-0 order");
> - return false;
> + return -EINVAL;
> }
> }
>
> @@ -3732,12 +3755,16 @@ bool folio_split_supported(struct folio *folio, unsigned int new_order,
> * here.
> */
> if ((split_type == SPLIT_TYPE_NON_UNIFORM || new_order) && folio_test_swapcache(folio)) {
> - VM_WARN_ONCE(warns,
> - "Cannot split swapcache folio to non-0 order");
> - return false;
> + return -EINVAL;
> }
>
> - return true;
> + if (is_huge_zero_folio(folio))
> + return -EINVAL;
> +
> + if (folio_test_writeback(folio))
> + return -EBUSY;
> +
> + return 0;
> }
>
> static int __folio_freeze_and_split_unmapped(struct folio *folio, unsigned int new_order,
> @@ -3922,7 +3949,6 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
> int remap_flags = 0;
> int extra_pins, ret;
> pgoff_t end = 0;
> - bool is_hzp;
>
> VM_WARN_ON_ONCE_FOLIO(!folio_test_locked(folio), folio);
> VM_WARN_ON_ONCE_FOLIO(!folio_test_large(folio), folio);
> @@ -3930,31 +3956,15 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
> if (folio != page_folio(split_at) || folio != page_folio(lock_at))
> return -EINVAL;
>
> - /*
> - * Folios that just got truncated cannot get split. Signal to the
> - * caller that there was a race.
> - *
> - * TODO: this will also currently refuse shmem folios that are in the
> - * swapcache.
> - */
> - if (!is_anon && !folio->mapping)
> - return -EBUSY;
> -
> if (new_order >= old_order)
> return -EINVAL;
>
> - if (!folio_split_supported(folio, new_order, split_type, /* warn = */ true))
> - return -EINVAL;
> -
> - is_hzp = is_huge_zero_folio(folio);
> - if (is_hzp) {
> - pr_warn_ratelimited("Called split_huge_page for huge zero page\n");
> - return -EBUSY;
> + ret = folio_check_splittable(folio, new_order, split_type);
> + if (ret) {
> + VM_WARN_ONCE(ret == -EINVAL, "Tried to split an unsplittable folio");
> + return ret;
> }
>
> - if (folio_test_writeback(folio))
> - return -EBUSY;
> -
> if (is_anon) {
> /*
> * The caller does not necessarily hold an mmap_lock that would
Otherwise,looks good!
Acked-by: Balbir Singh <balbirs@...dia.com>
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