Re: [PATCH] fixup: mm/huge_memory.c: introduce folio_split_unmapped

["David Hildenbrand (Red Hat)" <david@kernel.org>]

On 11/20/25 04:07, 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 checking for shmem_mapping() prior to calling
> shmem_uncharge(), shmem_mapping() returns false when CONFIG_SHMEM=n.
> 
> 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.
> 
> Add detailed documentation comments for folio_split_unmapped()
> 
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: Zi Yan <ziy@nvidia.com>
> Cc: Joshua Hahn <joshua.hahnjy@gmail.com>
> Cc: Rakie Kim <rakie.kim@sk.com>
> Cc: Byungchul Park <byungchul@sk.com>
> Cc: Gregory Price <gourry@gourry.net>
> Cc: Ying Huang <ying.huang@linux.alibaba.com>
> Cc: Alistair Popple <apopple@nvidia.com>
> Cc: Oscar Salvador <osalvador@suse.de>
> Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.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: Barry Song <baohua@kernel.org>
> Cc: Lyude Paul <lyude@redhat.com>
> Cc: Danilo Krummrich <dakr@kernel.org>
> Cc: David Airlie <airlied@gmail.com>
> Cc: Simona Vetter <simona@ffwll.ch>
> Cc: Ralph Campbell <rcampbell@nvidia.com>
> Cc: Mika Penttilä <mpenttil@redhat.com>
> Cc: Matthew Brost <matthew.brost@intel.com>
> Cc: Francois Dugast <francois.dugast@intel.com>
> 
> Signed-off-by: Balbir Singh <balbirs@nvidia.com>
> ---
>   mm/huge_memory.c | 32 ++++++++++++++++++++++----------
>   1 file changed, 22 insertions(+), 10 deletions(-)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 78a31a476ad3..c4267a0f74df 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -3751,6 +3751,7 @@ static int __folio_freeze_and_split_unmapped(struct folio *folio, unsigned int n
>   	int ret = 0;
>   	struct deferred_split *ds_queue;
>   
> +	VM_WARN_ON_ONCE(!mapping && end != 0);

You could drop the "!= 0"

>   	/* Prevent deferred_split_scan() touching ->_refcount */
>   	ds_queue = folio_split_queue_lock(folio);
>   	if (folio_ref_freeze(folio, 1 + extra_pins)) {
> @@ -3919,7 +3920,7 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
>   	int nr_shmem_dropped = 0;
>   	int remap_flags = 0;
>   	int extra_pins, ret;
> -	pgoff_t end;
> +	pgoff_t end = 0;
>   	bool is_hzp;
>   
>   	VM_WARN_ON_ONCE_FOLIO(!folio_test_locked(folio), folio);
> @@ -4049,7 +4050,7 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
>   
>   	local_irq_enable();
>   
> -	if (nr_shmem_dropped)
> +	if (mapping && shmem_mapping(mapping) && nr_shmem_dropped)
>   		shmem_uncharge(mapping->host, nr_shmem_dropped);

That looks questionable. We shouldn't add runtime check to handle 
buildtime things.

Likely what you want is instead

if (IS_ENABLED(CONFIG_SHMEM) && nr_shmem_dropped)
	shmem_uncharge()

>   
>   	if (!ret && is_anon && !folio_is_device_private(folio))
> @@ -4092,16 +4093,27 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
>   	return ret;
>   }
>   
> -/*
> - * This function is a helper for splitting folios that have already been unmapped.
> - * The use case is that the device or the CPU can refuse to migrate THP pages in
> - * the middle of migration, due to allocation issues on either side
> +/**
> + * folio_split_unmapped() - split a large anon folio that is already unmapped
> + * @folio: folio to split
> + * @new_order: the order of folios after split
> + *
> + * This function is a helper for splitting folios that have already been
> + * unmapped. The use case is that the device or the CPU can refuse to migrate
> + * THP pages in the middle of migration, due to allocation issues on either
> + * side.
> + *
> + * anon_vma_lock is not required to be held, mmap_read_lock() or
> + * mmap_write_lock() should be held. @folio is expected to be locked by the
> + * caller. device-private and non device-private folios are supported along
> + * with folios that are in the swapcache. @folio should also be unmapped and
> + * isolated from LRU (if applicable)
>    *
> - * The high level code is copied from __folio_split, since the pages are anonymous
> - * and are already isolated from the LRU, the code has been simplified to not
> - * burden __folio_split with unmapped sprinkled into the code.
> + * Upon return, the folio is not remapped, split folios are not added to LRU,
> + * free_folio_and_swap_cache() is not called, and new folios remain locked.
>    *
> - * None of the split folios are unlocked
> + * Return: 0 on success, -EAGAIN if the folio cannot be split (e.g., due to
> + *         insufficient reference count or extra pins).

Sounds much better to me, thanks.

-- 
Cheers

David