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); /* 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); 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). */ int folio_split_unmapped(struct folio *folio, unsigned int new_order) { -- 2.51.1
