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The purpose is to minimize the number of folios after the split. For example, if user wants to free the 3rd subpage in a order-9 folio, folio_split() will split the order-9 folio as: O-0, O-0, O-0, O-0, O-2, O-3, O-4, O-5, O-6, O-7, O-8 if it is anon, since anon folio does not support order-1 yet. ----------------------------------------------------------------- | | | | | | | | | |O-0|O-0|O-0|O-0| O-2 |...| O-7 | O-8 | | | | | | | | | | ----------------------------------------------------------------- O-1, O-0, O-0, O-2, O-3, O-4, O-5, O-6, O-7, O-9 if it is pagecache --------------------------------------------------------------- | | | | | | | | | O-1 |O-0|O-0| O-2 |...| O-7 | O-8 | | | | | | | | | --------------------------------------------------------------- It generates fewer folios (i.e., 11 or 10) than existing page split approach, which splits the order-9 to 512 order-0 folios. It also reduces the number of new xa_node needed during a pagecache folio split from 8 to 1, potentially decreasing the folio split failure rate due to memory constraints. folio_split() and existing split_huge_page_to_list_to_order() share the folio unmapping and remapping code in __folio_split() and the common backend split code in __split_unmapped_folio() using uniform_split variable to distinguish their operations. uniform_split_supported() and non_uniform_split_supported() are added to factor out check code and will be used outside __folio_split() in the following commit. Signed-off-by: Zi Yan Cc: Baolin Wang Cc: David Hildenbrand Cc: Hugh Dickins Cc: John Hubbard Cc: Kefeng Wang Cc: Kirill A. Shuemov Cc: Matthew Wilcox Cc: Miaohe Lin Cc: Ryan Roberts Cc: Yang Shi Cc: Yu Zhao Cc: Kairui Song --- mm/huge_memory.c | 170 +++++++++++++++++++++++++++++++++++------------ 1 file changed, 128 insertions(+), 42 deletions(-) diff --git a/mm/huge_memory.c b/mm/huge_memory.c index 871c260163f1..3e05e62fdccb 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -3863,12 +3863,85 @@ static int __split_unmapped_folio(struct folio *folio, int new_order, return ret; } +static bool non_uniform_split_supported(struct folio *folio, unsigned int new_order, + bool warns) +{ + if (folio_test_anon(folio)) { + /* order-1 is not supported for anonymous THP. */ + VM_WARN_ONCE(warns && new_order == 1, + "Cannot split to order-1 folio"); + return new_order != 1; + } else if (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) && + !mapping_large_folio_support(folio->mapping)) { + /* + * No split if the file system does not support large folio. + * Note that we might still have THPs in such mappings due to + * CONFIG_READ_ONLY_THP_FOR_FS. But in that case, the mapping + * does not actually support large folios properly. + */ + VM_WARN_ONCE(warns, + "Cannot split file folio to non-0 order"); + return false; + } + + /* Only swapping a whole PMD-mapped folio is supported */ + if (folio_test_swapcache(folio)) { + VM_WARN_ONCE(warns, + "Cannot split swapcache folio to non-0 order"); + return false; + } + + return true; +} + +/* See comments in non_uniform_split_supported() */ +static bool uniform_split_supported(struct folio *folio, unsigned int new_order, + bool warns) +{ + if (folio_test_anon(folio)) { + VM_WARN_ONCE(warns && new_order == 1, + "Cannot split to order-1 folio"); + return new_order != 1; + } else if (new_order) { + if (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) && + !mapping_large_folio_support(folio->mapping)) { + VM_WARN_ONCE(warns, + "Cannot split file folio to non-0 order"); + return false; + } + } + + if (new_order && folio_test_swapcache(folio)) { + VM_WARN_ONCE(warns, + "Cannot split swapcache folio to non-0 order"); + return false; + } + + return true; +} + +/* + * __folio_split: split a folio at @split_at to a @new_order folio + * @folio: folio to split + * @new_order: the order of the new folio + * @split_at: a page within the new folio + * @lock_at: a page within @folio to be left locked to caller + * @list: after-split folios will be put on it if non NULL + * @uniform_split: perform uniform split or not (non-uniform split) + * + * It calls __split_unmapped_folio() to perform uniform and non-uniform split. + * It is in charge of checking whether the split is supported or not and + * preparing @folio for __split_unmapped_folio(). + * + * return: 0: successful, <0 failed (if -ENOMEM is returned, @folio might be + * split but not to @new_order, the caller needs to check) + */ static int __folio_split(struct folio *folio, unsigned int new_order, - struct page *page, struct list_head *list) + struct page *split_at, struct page *lock_at, + struct list_head *list, bool uniform_split) { struct deferred_split *ds_queue = get_deferred_split_queue(folio); - /* reset xarray order to new order after split */ - XA_STATE_ORDER(xas, &folio->mapping->i_pages, folio->index, new_order); + XA_STATE(xas, &folio->mapping->i_pages, folio->index); bool is_anon = folio_test_anon(folio); struct address_space *mapping = NULL; struct anon_vma *anon_vma = NULL; @@ -3880,32 +3953,17 @@ static int __folio_split(struct folio *folio, unsigned int new_order, VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio); VM_BUG_ON_FOLIO(!folio_test_large(folio), folio); + if (folio != page_folio(split_at) || folio != page_folio(lock_at)) + return -EINVAL; + if (new_order >= folio_order(folio)) return -EINVAL; - if (is_anon) { - /* order-1 is not supported for anonymous THP. */ - if (new_order == 1) { - VM_WARN_ONCE(1, "Cannot split to order-1 folio"); - return -EINVAL; - } - } else if (new_order) { - /* - * No split if the file system does not support large folio. - * Note that we might still have THPs in such mappings due to - * CONFIG_READ_ONLY_THP_FOR_FS. But in that case, the mapping - * does not actually support large folios properly. - */ - if (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) && - !mapping_large_folio_support(folio->mapping)) { - VM_WARN_ONCE(1, - "Cannot split file folio to non-0 order"); - return -EINVAL; - } - } + if (uniform_split && !uniform_split_supported(folio, new_order, true)) + return -EINVAL; - /* Only swapping a whole PMD-mapped folio is supported */ - if (folio_test_swapcache(folio) && new_order) + if (!uniform_split && + !non_uniform_split_supported(folio, new_order, true)) return -EINVAL; is_hzp = is_huge_zero_folio(folio); @@ -3967,21 +4025,24 @@ static int __folio_split(struct folio *folio, unsigned int new_order, goto out; } - xas_split_alloc(&xas, folio, folio_order(folio), gfp); - if (xas_error(&xas)) { - ret = xas_error(&xas); - goto out; + if (uniform_split) { + xas_set_order(&xas, folio->index, new_order); + xas_split_alloc(&xas, folio, folio_order(folio), gfp); + if (xas_error(&xas)) { + ret = xas_error(&xas); + goto out; + } } anon_vma = NULL; i_mmap_lock_read(mapping); /* - *__split_huge_page() may need to trim off pages beyond EOF: - * but on 32-bit, i_size_read() takes an irq-unsafe seqlock, - * which cannot be nested inside the page tree lock. So note - * end now: i_size itself may be changed at any moment, but - * folio lock is good enough to serialize the trimming. + *__split_unmapped_folio() may need to trim off pages beyond + * EOF: but on 32-bit, i_size_read() takes an irq-unsafe + * seqlock, which cannot be nested inside the page tree lock. + * So note end now: i_size itself may be changed at any moment, + * but folio lock is good enough to serialize the trimming. */ end = DIV_ROUND_UP(i_size_read(mapping->host), PAGE_SIZE); if (shmem_mapping(mapping)) @@ -4035,7 +4096,6 @@ static int __folio_split(struct folio *folio, unsigned int new_order, if (mapping) { int nr = folio_nr_pages(folio); - xas_split(&xas, folio, folio_order(folio)); if (folio_test_pmd_mappable(folio) && new_order < HPAGE_PMD_ORDER) { if (folio_test_swapbacked(folio)) { @@ -4049,12 +4109,9 @@ static int __folio_split(struct folio *folio, unsigned int new_order, } } - if (is_anon) { - mod_mthp_stat(order, MTHP_STAT_NR_ANON, -1); - mod_mthp_stat(new_order, MTHP_STAT_NR_ANON, 1 << (order - new_order)); - } - __split_huge_page(page, list, end, new_order); - ret = 0; + ret = __split_unmapped_folio(folio, new_order, + split_at, lock_at, list, end, &xas, mapping, + uniform_split); } else { spin_unlock(&ds_queue->split_queue_lock); fail: @@ -4132,7 +4189,36 @@ int split_huge_page_to_list_to_order(struct page *page, struct list_head *list, { struct folio *folio = page_folio(page); - return __folio_split(folio, new_order, page, list); + return __folio_split(folio, new_order, &folio->page, page, list, true); +} + +/* + * folio_split: split a folio at @split_at to a @new_order folio + * @folio: folio to split + * @new_order: the order of the new folio + * @split_at: a page within the new folio + * + * return: 0: successful, <0 failed (if -ENOMEM is returned, @folio might be + * split but not to @new_order, the caller needs to check) + * + * It has the same prerequisites and returns as + * split_huge_page_to_list_to_order(). + * + * Split a folio at @split_at to a new_order folio, leave the + * remaining subpages of the original folio as large as possible. For example, + * in the case of splitting an order-9 folio at its third order-3 subpages to + * an order-3 folio, there are 2^(9-3)=64 order-3 subpages in the order-9 folio. + * After the split, there will be a group of folios with different orders and + * the new folio containing @split_at is marked in bracket: + * [order-4, {order-3}, order-3, order-5, order-6, order-7, order-8]. + * + * After split, folio is left locked for caller. + */ +static int folio_split(struct folio *folio, unsigned int new_order, + struct page *split_at, struct list_head *list) +{ + return __folio_split(folio, new_order, split_at, &folio->page, list, + false); } int min_order_for_split(struct folio *folio)