On 11/20/25 20:09, David Hildenbrand (Red Hat) wrote:
> 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 <[email protected]>
>> Cc: David Hildenbrand <[email protected]>
>> Cc: Zi Yan <[email protected]>
>> Cc: Joshua Hahn <[email protected]>
>> Cc: Rakie Kim <[email protected]>
>> Cc: Byungchul Park <[email protected]>
>> Cc: Gregory Price <[email protected]>
>> Cc: Ying Huang <[email protected]>
>> Cc: Alistair Popple <[email protected]>
>> Cc: Oscar Salvador <[email protected]>
>> Cc: Lorenzo Stoakes <[email protected]>
>> Cc: Baolin Wang <[email protected]>
>> Cc: "Liam R. Howlett" <[email protected]>
>> Cc: Nico Pache <[email protected]>
>> Cc: Ryan Roberts <[email protected]>
>> Cc: Dev Jain <[email protected]>
>> Cc: Barry Song <[email protected]>
>> Cc: Lyude Paul <[email protected]>
>> Cc: Danilo Krummrich <[email protected]>
>> Cc: David Airlie <[email protected]>
>> Cc: Simona Vetter <[email protected]>
>> Cc: Ralph Campbell <[email protected]>
>> Cc: Mika Penttilä <[email protected]>
>> Cc: Matthew Brost <[email protected]>
>> Cc: Francois Dugast <[email protected]>
>>
>> Signed-off-by: Balbir Singh <[email protected]>
>> ---
>> 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"
Ack
VM_WARN_ONE(!mapping && end);
>
>> /* 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()
>
shmem_mapping() returns false for CONFIG_SHMEM=n and shmem_mapping() checks
that the mapping
is indeed for shmem ops before uncharging. Happy to change it if you like,
your version is more readable
>> 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.
>
Thanks
Balbir
