[PATCH] mm/huge_memory: Fix outdated comment about freeing subpages in __folio_split

[PATCH] mm/huge_memory: Fix outdated comment about freeing subpages in __folio_split

[Barry Song (Xiaomi)]

The comment appears to be outdated. First, add_to_swap() no longer
exists after Kairui’s commit b487a2da3575 ("mm, swap: simplify folio
swap allocation"). Second, partially zapped folios are now always
split before folio_alloc_swap() to avoid extra I/O, following Ryan’s
commit 5ed890ce5147 ("mm: vmscan: avoid split during
shrink_folio_list()").
Fix this by making the description more generic.

Cc: David Hildenbrand <david@kernel.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: "Liam R. Howlett" <Liam.Howlett@oracle.com>
Cc: Nico Pache <npache@redhat.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Chris Li <chrisl@kernel.org>
Cc: Kairui Song <kasong@tencent.com>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: Youngjun Park <youngjun.park@lge.com>
Signed-off-by: Barry Song (Xiaomi) <baohua@kernel.org>
---
 mm/huge_memory.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 970e077019b7..4586f3ccb133 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -4190,11 +4190,10 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
 
 		folio_unlock(new_folio);
 		/*
-		 * Subpages may be freed if there wasn't any mapping
-		 * like if add_to_swap() is running on a lru page that
-		 * had its mapping zapped. And freeing these pages
-		 * requires taking the lru_lock so we do the put_page
-		 * of the tail pages after the split is complete.
+		 * Subpages whose mapping has been zapped may be freed
+		 * earlier, but freeing them requires taking the
+		 * lru_lock, so we defer put_page() on tail pages until
+		 * after the split completes.
 		 */
 		free_folio_and_swap_cache(new_folio);
 	}
-- 
2.39.3 (Apple Git-146)

Re: [PATCH] mm/huge_memory: Fix outdated comment about freeing subpages in __folio_split

[David Hildenbrand (Arm)]

On 4/23/26 05:49, Barry Song (Xiaomi) wrote:
> The comment appears to be outdated. First, add_to_swap() no longer
> exists after Kairui’s commit b487a2da3575 ("mm, swap: simplify folio
> swap allocation"). Second, partially zapped folios are now always
> split before folio_alloc_swap() to avoid extra I/O, following Ryan’s
> commit 5ed890ce5147 ("mm: vmscan: avoid split during
> shrink_folio_list()").

Not necessarily. We have some scenarios where we have partially-mapped folios
not detected as such as of today (with CONFIG_NO_PAGE_MAPCOUNT).

> Fix this by making the description more generic.
> 
> Cc: David Hildenbrand <david@kernel.org>
> Cc: Lorenzo Stoakes <ljs@kernel.org>
> Cc: Zi Yan <ziy@nvidia.com>
> Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
> Cc: "Liam R. Howlett" <Liam.Howlett@oracle.com>
> Cc: Nico Pache <npache@redhat.com>
> Cc: Ryan Roberts <ryan.roberts@arm.com>
> Cc: Dev Jain <dev.jain@arm.com>
> Cc: Lance Yang <lance.yang@linux.dev>
> Cc: Chris Li <chrisl@kernel.org>
> Cc: Kairui Song <kasong@tencent.com>
> Cc: Kemeng Shi <shikemeng@huaweicloud.com>
> Cc: Nhat Pham <nphamcs@gmail.com>
> Cc: Baoquan He <bhe@redhat.com>
> Cc: Youngjun Park <youngjun.park@lge.com>
> Signed-off-by: Barry Song (Xiaomi) <baohua@kernel.org>
> ---
>  mm/huge_memory.c | 9 ++++-----
>  1 file changed, 4 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 970e077019b7..4586f3ccb133 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -4190,11 +4190,10 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
>  
>  		folio_unlock(new_folio);
>  		/*
> -		 * Subpages may be freed if there wasn't any mapping
> -		 * like if add_to_swap() is running on a lru page that
> -		 * had its mapping zapped. And freeing these pages
> -		 * requires taking the lru_lock so we do the put_page
> -		 * of the tail pages after the split is complete.
> +		 * Subpages whose mapping has been zapped may be freed
> +		 * earlier, but freeing them requires taking the
> +		 * lru_lock, so we defer put_page() on tail pages until
> +		 * after the split completes.
>  		 */
>  		free_folio_and_swap_cache(new_folio);
>  	}

Acked-by: David Hildenbrand (Arm) <david@kernel.org>


-- 
Cheers,

David

Re: [PATCH] mm/huge_memory: Fix outdated comment about freeing subpages in __folio_split

[Barry Song]

On Thu, Apr 23, 2026 at 8:36 PM David Hildenbrand (Arm)
<david@kernel.org> wrote:
>
> On 4/23/26 05:49, Barry Song (Xiaomi) wrote:
> > The comment appears to be outdated. First, add_to_swap() no longer
> > exists after Kairui’s commit b487a2da3575 ("mm, swap: simplify folio
> > swap allocation"). Second, partially zapped folios are now always
> > split before folio_alloc_swap() to avoid extra I/O, following Ryan’s
> > commit 5ed890ce5147 ("mm: vmscan: avoid split during
> > shrink_folio_list()").
>
> Not necessarily. We have some scenarios where we have partially-mapped folios
> not detected as such as of today (with CONFIG_NO_PAGE_MAPCOUNT).

Thanks, david! would it make more sense to add “when
CONFIG_PAGE_MAPCOUNT is enabled” at the end of the
sentence? I mean:
"
following Ryan’s
commit 5ed890ce5147 ("mm: vmscan: avoid split during
shrink_folio_list()") when CONFIG_PAGE_MAPCOUNT is enabled.
"

>
> > Fix this by making the description more generic.
> >
> > Cc: David Hildenbrand <david@kernel.org>
> > Cc: Lorenzo Stoakes <ljs@kernel.org>
> > Cc: Zi Yan <ziy@nvidia.com>
> > Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
> > Cc: "Liam R. Howlett" <Liam.Howlett@oracle.com>
> > Cc: Nico Pache <npache@redhat.com>
> > Cc: Ryan Roberts <ryan.roberts@arm.com>
> > Cc: Dev Jain <dev.jain@arm.com>
> > Cc: Lance Yang <lance.yang@linux.dev>
> > Cc: Chris Li <chrisl@kernel.org>
> > Cc: Kairui Song <kasong@tencent.com>
> > Cc: Kemeng Shi <shikemeng@huaweicloud.com>
> > Cc: Nhat Pham <nphamcs@gmail.com>
> > Cc: Baoquan He <bhe@redhat.com>
> > Cc: Youngjun Park <youngjun.park@lge.com>
> > Signed-off-by: Barry Song (Xiaomi) <baohua@kernel.org>
> > ---
> >  mm/huge_memory.c | 9 ++++-----
> >  1 file changed, 4 insertions(+), 5 deletions(-)
> >
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index 970e077019b7..4586f3ccb133 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -4190,11 +4190,10 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
> >
> >               folio_unlock(new_folio);
> >               /*
> > -              * Subpages may be freed if there wasn't any mapping
> > -              * like if add_to_swap() is running on a lru page that
> > -              * had its mapping zapped. And freeing these pages
> > -              * requires taking the lru_lock so we do the put_page
> > -              * of the tail pages after the split is complete.
> > +              * Subpages whose mapping has been zapped may be freed
> > +              * earlier, but freeing them requires taking the
> > +              * lru_lock, so we defer put_page() on tail pages until
> > +              * after the split completes.
> >                */
> >               free_folio_and_swap_cache(new_folio);
> >       }
>
> Acked-by: David Hildenbrand (Arm) <david@kernel.org>

Thanks for reviewing!

Best Regards,
Barry

Re: [PATCH] mm/huge_memory: Fix outdated comment about freeing subpages in __folio_split

[David Hildenbrand (Arm)]

On 4/26/26 22:42, Barry Song wrote:
> On Thu, Apr 23, 2026 at 8:36 PM David Hildenbrand (Arm)
> <david@kernel.org> wrote:
>>
>> On 4/23/26 05:49, Barry Song (Xiaomi) wrote:
>>> The comment appears to be outdated. First, add_to_swap() no longer
>>> exists after Kairui’s commit b487a2da3575 ("mm, swap: simplify folio
>>> swap allocation"). Second, partially zapped folios are now always
>>> split before folio_alloc_swap() to avoid extra I/O, following Ryan’s
>>> commit 5ed890ce5147 ("mm: vmscan: avoid split during
>>> shrink_folio_list()").
>>
>> Not necessarily. We have some scenarios where we have partially-mapped folios
>> not detected as such as of today (with CONFIG_NO_PAGE_MAPCOUNT).
> 
> Thanks, david! would it make more sense to add “when
> CONFIG_PAGE_MAPCOUNT is enabled” at the end of the
> sentence? 

Better to phrase it in a way that doesn't even require these details :)

Do we even need this detail in the patch description?

-- 
Cheers,

David

Re: [PATCH] mm/huge_memory: Fix outdated comment about freeing subpages in __folio_split

[Barry Song]

On Mon, Apr 27, 2026 at 3:42 PM David Hildenbrand (Arm)
<david@kernel.org> wrote:
>
> On 4/26/26 22:42, Barry Song wrote:
> > On Thu, Apr 23, 2026 at 8:36 PM David Hildenbrand (Arm)
> > <david@kernel.org> wrote:
> >>
> >> On 4/23/26 05:49, Barry Song (Xiaomi) wrote:
> >>> The comment appears to be outdated. First, add_to_swap() no longer
> >>> exists after Kairui’s commit b487a2da3575 ("mm, swap: simplify folio
> >>> swap allocation"). Second, partially zapped folios are now always
> >>> split before folio_alloc_swap() to avoid extra I/O, following Ryan’s
> >>> commit 5ed890ce5147 ("mm: vmscan: avoid split during
> >>> shrink_folio_list()").
> >>
> >> Not necessarily. We have some scenarios where we have partially-mapped folios
> >> not detected as such as of today (with CONFIG_NO_PAGE_MAPCOUNT).
> >
> > Thanks, david! would it make more sense to add “when
> > CONFIG_PAGE_MAPCOUNT is enabled” at the end of the
> > sentence?
>
> Better to phrase it in a way that doesn't even require these details :)
>
> Do we even need this detail in the patch description?

What about:

The comment appears to be outdated. add_to_swap() no longer exists,
and the explanation of why we need to call put_page() after
splitting could be made more general.

Best Regards
Barry

Re: [PATCH] mm/huge_memory: Fix outdated comment about freeing subpages in __folio_split

[David Hildenbrand (Arm)]

On 4/28/26 12:05, Barry Song wrote:
> On Mon, Apr 27, 2026 at 3:42 PM David Hildenbrand (Arm)
> <david@kernel.org> wrote:
>>
>> On 4/26/26 22:42, Barry Song wrote:
>>> On Thu, Apr 23, 2026 at 8:36 PM David Hildenbrand (Arm)
>>> <david@kernel.org> wrote:
>>>
>>> Thanks, david! would it make more sense to add “when
>>> CONFIG_PAGE_MAPCOUNT is enabled” at the end of the
>>> sentence?
>>
>> Better to phrase it in a way that doesn't even require these details :)
>>
>> Do we even need this detail in the patch description?
> 
> What about:
> 
> The comment appears to be outdated. add_to_swap() no longer exists,
> and the explanation of why we need to call put_page() after
> splitting could be made more general.

Jup :)

-- 
Cheers,

David

Re: [PATCH] mm/huge_memory: Fix outdated comment about freeing subpages in __folio_split

[Barry Song]

On Tue, Apr 28, 2026 at 6:08 PM David Hildenbrand (Arm)
<david@kernel.org> wrote:
>
> On 4/28/26 12:05, Barry Song wrote:
> > On Mon, Apr 27, 2026 at 3:42 PM David Hildenbrand (Arm)
> > <david@kernel.org> wrote:
> >>
> >> On 4/26/26 22:42, Barry Song wrote:
> >>> On Thu, Apr 23, 2026 at 8:36 PM David Hildenbrand (Arm)
> >>> <david@kernel.org> wrote:
> >>>
> >>> Thanks, david! would it make more sense to add “when
> >>> CONFIG_PAGE_MAPCOUNT is enabled” at the end of the
> >>> sentence?
> >>
> >> Better to phrase it in a way that doesn't even require these details :)
> >>
> >> Do we even need this detail in the patch description?
> >
> > What about:
> >
> > The comment appears to be outdated. add_to_swap() no longer exists,
> > and the explanation of why we need to call put_page() after
> > splitting could be made more general.
>
> Jup :)

Thanks!
Hi Andrew, would you like to make the change in the
changelog, or would you prefer that I send a new version?

Thanks
Barry

Re: [PATCH] mm/huge_memory: Fix outdated comment about freeing subpages in __folio_split

[Andrew Morton]

On Tue, 28 Apr 2026 18:32:40 +0800 Barry Song <baohua@kernel.org> wrote:

> On Tue, Apr 28, 2026 at 6:08 PM David Hildenbrand (Arm)
> <david@kernel.org> wrote:
> >
> > On 4/28/26 12:05, Barry Song wrote:
> > > On Mon, Apr 27, 2026 at 3:42 PM David Hildenbrand (Arm)
> > > <david@kernel.org> wrote:
> > >>
> > >> On 4/26/26 22:42, Barry Song wrote:
> > >>> On Thu, Apr 23, 2026 at 8:36 PM David Hildenbrand (Arm)
> > >>> <david@kernel.org> wrote:
> > >>>
> > >>> Thanks, david! would it make more sense to add “when
> > >>> CONFIG_PAGE_MAPCOUNT is enabled” at the end of the
> > >>> sentence?
> > >>
> > >> Better to phrase it in a way that doesn't even require these details :)
> > >>
> > >> Do we even need this detail in the patch description?
> > >
> > > What about:
> > >
> > > The comment appears to be outdated. add_to_swap() no longer exists,
> > > and the explanation of why we need to call put_page() after
> > > splitting could be made more general.
> >
> > Jup :)
> 
> Thanks!
> Hi Andrew, would you like to make the change in the
> changelog, or would you prefer that I send a new version?

This?

From: "Barry Song (Xiaomi)" <baohua@kernel.org>
Subject: mm/huge_memory: fix outdated comment about freeing subpages in __folio_split
Date: Thu, 23 Apr 2026 11:49:17 +0800

The comment appears to be outdated.  add_to_swap() no longer exists,
and the explanation of why we need to call put_page() after splitting
could be made more general.

Link: https://lore.kernel.org/20260423034917.8234-1-baohua@kernel.org
Signed-off-by: Barry Song (Xiaomi) <baohua@kernel.org>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Acked-by: Zi Yan <ziy@nvidia.com>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Nico Pache <npache@redhat.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Chris Li <chrisl@kernel.org>
Cc: Kairui Song <kasong@tencent.com>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: Youngjun Park <youngjun.park@lge.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/huge_memory.c |    9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

--- a/mm/huge_memory.c~mm-huge_memory-fix-outdated-comment-about-freeing-subpages-in-__folio_split
+++ a/mm/huge_memory.c
@@ -4190,11 +4190,10 @@ fail:
 
 		folio_unlock(new_folio);
 		/*
-		 * Subpages may be freed if there wasn't any mapping
-		 * like if add_to_swap() is running on a lru page that
-		 * had its mapping zapped. And freeing these pages
-		 * requires taking the lru_lock so we do the put_page
-		 * of the tail pages after the split is complete.
+		 * Subpages whose mapping has been zapped may be freed
+		 * earlier, but freeing them requires taking the
+		 * lru_lock, so we defer put_page() on tail pages until
+		 * after the split completes.
 		 */
 		free_folio_and_swap_cache(new_folio);
 	}
_

Re: [PATCH] mm/huge_memory: Fix outdated comment about freeing subpages in __folio_split

[Barry Song]

On Tue, Apr 28, 2026 at 9:01 PM Andrew Morton <akpm@linux-foundation.org> wrote:
>
> On Tue, 28 Apr 2026 18:32:40 +0800 Barry Song <baohua@kernel.org> wrote:
>
> > On Tue, Apr 28, 2026 at 6:08 PM David Hildenbrand (Arm)
> > <david@kernel.org> wrote:
> > >
> > > On 4/28/26 12:05, Barry Song wrote:
> > > > On Mon, Apr 27, 2026 at 3:42 PM David Hildenbrand (Arm)
> > > > <david@kernel.org> wrote:
> > > >>
> > > >> On 4/26/26 22:42, Barry Song wrote:
> > > >>> On Thu, Apr 23, 2026 at 8:36 PM David Hildenbrand (Arm)
> > > >>> <david@kernel.org> wrote:
> > > >>>
> > > >>> Thanks, david! would it make more sense to add “when
> > > >>> CONFIG_PAGE_MAPCOUNT is enabled” at the end of the
> > > >>> sentence?
> > > >>
> > > >> Better to phrase it in a way that doesn't even require these details :)
> > > >>
> > > >> Do we even need this detail in the patch description?
> > > >
> > > > What about:
> > > >
> > > > The comment appears to be outdated. add_to_swap() no longer exists,
> > > > and the explanation of why we need to call put_page() after
> > > > splitting could be made more general.
> > >
> > > Jup :)
> >
> > Thanks!
> > Hi Andrew, would you like to make the change in the
> > changelog, or would you prefer that I send a new version?
>
> This?

Yes, exactly.

Thanks very much!

>
> From: "Barry Song (Xiaomi)" <baohua@kernel.org>
> Subject: mm/huge_memory: fix outdated comment about freeing subpages in __folio_split
> Date: Thu, 23 Apr 2026 11:49:17 +0800
>
> The comment appears to be outdated.  add_to_swap() no longer exists,
> and the explanation of why we need to call put_page() after splitting
> could be made more general.

Re: [PATCH] mm/huge_memory: Fix outdated comment about freeing subpages in __folio_split

[Zi Yan]

On 22 Apr 2026, at 23:49, Barry Song (Xiaomi) wrote:

> The comment appears to be outdated. First, add_to_swap() no longer
> exists after Kairui’s commit b487a2da3575 ("mm, swap: simplify folio
> swap allocation"). Second, partially zapped folios are now always
> split before folio_alloc_swap() to avoid extra I/O, following Ryan’s
> commit 5ed890ce5147 ("mm: vmscan: avoid split during
> shrink_folio_list()").
> Fix this by making the description more generic.
>
> Cc: David Hildenbrand <david@kernel.org>
> Cc: Lorenzo Stoakes <ljs@kernel.org>
> Cc: Zi Yan <ziy@nvidia.com>
> Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
> Cc: "Liam R. Howlett" <Liam.Howlett@oracle.com>
> Cc: Nico Pache <npache@redhat.com>
> Cc: Ryan Roberts <ryan.roberts@arm.com>
> Cc: Dev Jain <dev.jain@arm.com>
> Cc: Lance Yang <lance.yang@linux.dev>
> Cc: Chris Li <chrisl@kernel.org>
> Cc: Kairui Song <kasong@tencent.com>
> Cc: Kemeng Shi <shikemeng@huaweicloud.com>
> Cc: Nhat Pham <nphamcs@gmail.com>
> Cc: Baoquan He <bhe@redhat.com>
> Cc: Youngjun Park <youngjun.park@lge.com>
> Signed-off-by: Barry Song (Xiaomi) <baohua@kernel.org>
> ---
>  mm/huge_memory.c | 9 ++++-----
>  1 file changed, 4 insertions(+), 5 deletions(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 970e077019b7..4586f3ccb133 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -4190,11 +4190,10 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
>
>  		folio_unlock(new_folio);
>  		/*
> -		 * Subpages may be freed if there wasn't any mapping
> -		 * like if add_to_swap() is running on a lru page that
> -		 * had its mapping zapped. And freeing these pages
> -		 * requires taking the lru_lock so we do the put_page
> -		 * of the tail pages after the split is complete.
> +		 * Subpages whose mapping has been zapped may be freed
> +		 * earlier, but freeing them requires taking the
> +		 * lru_lock, so we defer put_page() on tail pages until
> +		 * after the split completes.
>  		 */
>  		free_folio_and_swap_cache(new_folio);
>  	}
> -- 
> 2.39.3 (Apple Git-146)

Acked-by: Zi Yan <ziy@nvidia.com>

--
Best Regards,
Yan, Zi