linux-kernel - Re: [PATCH v2] fixup: mm/huge_memory.c: introduce folio_split

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Message-ID: <6757c538-d08b-4cad-95cc-772dfcabf909@nvidia.com>
Date: Fri, 21 Nov 2025 01:40:29 +1100
From: Balbir Singh <balbirs@...dia.com>
To: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 dri-devel@...ts.freedesktop.org
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 David Hildenbrand <david@...hat.com>, Zi Yan <ziy@...dia.com>,
 Joshua Hahn <joshua.hahnjy@...il.com>, Rakie Kim <rakie.kim@...com>,
 Byungchul Park <byungchul@...com>, Gregory Price <gourry@...rry.net>,
 Ying Huang <ying.huang@...ux.alibaba.com>,
 Alistair Popple <apopple@...dia.com>, Oscar Salvador <osalvador@...e.de>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 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>, Lyude Paul <lyude@...hat.com>,
 Danilo Krummrich <dakr@...nel.org>, David Airlie <airlied@...il.com>,
 Simona Vetter <simona@...ll.ch>, Ralph Campbell <rcampbell@...dia.com>,
 Mika Penttilä <mpenttil@...hat.com>,
 Matthew Brost <matthew.brost@...el.com>,
 Francois Dugast <francois.dugast@...el.com>
Subject: Re: [PATCH v2] fixup: mm/huge_memory.c: introduce
 folio_split_unmapped

On 11/21/25 00:42, Balbir Singh wrote:
> Code refactoring of __folio_split() via helper
> __folio_freeze_and_split_unmapped() caused a regression with clang-20
> with CONFIG_SHMEM=n, the compiler was not able to optimize away the
> call to shmem_uncharge() due to changes in nr_shmem_dropped.
> Fix this by adding a stub function for shmem_uncharge when
> CONFIG_SHMEM is not defined.
> 
> smatch also complained about parameter end being used without
> initialization, which is a false positive, but keep the tool happy
> by sending in initialized parameters. end is initialized to 0.
> smatch still complains about mapping being NULL and nr_shmem_dropped
> may not be 0, but that is not true prior to or after the changes.
> 
> Add detailed documentation comments for folio_split_unmapped()
> 
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: David Hildenbrand <david@...hat.com>
> Cc: Zi Yan <ziy@...dia.com>
> Cc: Joshua Hahn <joshua.hahnjy@...il.com>
> Cc: Rakie Kim <rakie.kim@...com>
> Cc: Byungchul Park <byungchul@...com>
> Cc: Gregory Price <gourry@...rry.net>
> Cc: Ying Huang <ying.huang@...ux.alibaba.com>
> Cc: Alistair Popple <apopple@...dia.com>
> Cc: Oscar Salvador <osalvador@...e.de>
> Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> Cc: Baolin Wang <baolin.wang@...ux.alibaba.com>
> Cc: "Liam R. Howlett" <Liam.Howlett@...cle.com>
> Cc: Nico Pache <npache@...hat.com>
> Cc: Ryan Roberts <ryan.roberts@....com>
> Cc: Dev Jain <dev.jain@....com>
> Cc: Barry Song <baohua@...nel.org>
> Cc: Lyude Paul <lyude@...hat.com>
> Cc: Danilo Krummrich <dakr@...nel.org>
> Cc: David Airlie <airlied@...il.com>
> Cc: Simona Vetter <simona@...ll.ch>
> Cc: Ralph Campbell <rcampbell@...dia.com>
> Cc: Mika Penttilä <mpenttil@...hat.com>
> Cc: Matthew Brost <matthew.brost@...el.com>
> Cc: Francois Dugast <francois.dugast@...el.com>
> 
> Suggested-by: David Hildenbrand <david@...hat.com>
> Signed-off-by: Balbir Singh <balbirs@...dia.com>
> ---
> This fixup should be squashed into the patch "mm/huge_memory.c:
> introduce folio_split_unmapped" in mm/mm-unstable
> 
>  include/linux/shmem_fs.h |  6 +++++-
>  mm/huge_memory.c         | 30 +++++++++++++++++++++---------
>  2 files changed, 26 insertions(+), 10 deletions(-)
> 
> diff --git a/include/linux/shmem_fs.h b/include/linux/shmem_fs.h
> index 5b368f9549d6..7a412dd6eb4f 100644
> --- a/include/linux/shmem_fs.h
> +++ b/include/linux/shmem_fs.h
> @@ -136,11 +136,16 @@ static inline bool shmem_hpage_pmd_enabled(void)
>  
>  #ifdef CONFIG_SHMEM
>  extern unsigned long shmem_swap_usage(struct vm_area_struct *vma);
> +extern void shmem_uncharge(struct inode *inode, long pages);
>  #else
>  static inline unsigned long shmem_swap_usage(struct vm_area_struct *vma)
>  {
>  	return 0;
>  }
> +
> +static void shmem_uncharge(struct inode *inode, long pages)
> +{
> +}
>  #endif

This is missing an inline attribute, my compilation tests did not catch it, 
but I'll do a new version tomorrow

Balbir

Balbir