The patch titled
Subject: mm/huge_memory: improve split_huge_page_to_list_to_order() return value documentation
has been added to the -mm mm-unstable branch. Its filename is
mm-huge_memory-improve-split_huge_page_to_list_to_order-return-value-documentation.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-huge_memory-improve-split_huge_page_to_list_to_order-return-value-documentation.patch
This patch will later appear in the mm-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: David Hildenbrand <david@xxxxxxxxxx>
Subject: mm/huge_memory: improve split_huge_page_to_list_to_order() return value documentation
Date: Thu, 18 Apr 2024 17:18:34 +0200
The documentation is wrong and relying on it almost resulted in BUGs in
new callers: we return -EAGAIN on unexpected folio references, not -EBUSY.
Let's fix that and also document which other return values we can
currently see and why they could happen.
Link: https://lkml.kernel.org/r/20240418151834.216557-1-david@xxxxxxxxxx
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
Cc: John Hubbard <jhubbard@xxxxxxxxxx>
Cc: Zi Yan <ziy@xxxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/huge_memory.c | 13 ++++++++++---
1 file changed, 10 insertions(+), 3 deletions(-)
--- a/mm/huge_memory.c~mm-huge_memory-improve-split_huge_page_to_list_to_order-return-value-documentation
+++ a/mm/huge_memory.c
@@ -2956,7 +2956,7 @@ bool can_split_folio(struct folio *folio
*
* 3) The folio must not be pinned. Any unexpected folio references, including
* GUP pins, will result in the folio not getting split; instead, the caller
- * will receive an -EBUSY.
+ * will receive an -EAGAIN.
*
* 4) @new_order > 1, usually. Splitting to order-1 anonymous folios is not
* supported for non-file-backed folios, because folio->_deferred_list, which
@@ -2975,8 +2975,15 @@ bool can_split_folio(struct folio *folio
*
* Returns 0 if the huge page was split successfully.
*
- * Returns -EBUSY if @page's folio is pinned, or if the anon_vma disappeared
- * from under us.
+ * Returns -EAGAIN if the folio has unexpected reference (e.g., GUP).
+ *
+ * Returns -EBUSY when trying to split the huge zeropage, if the folio is
+ * under writeback, if fs-specific folio metadata cannot currently be
+ * released, or if some unexpected race happened (e.g., anon VMA disappeared,
+ * truncation).
+ *
+ * Returns -EINVAL when trying to split to an order that is incompatible
+ * with the folio. Splitting to order 0 is compatible with all folios.
*/
int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
unsigned int new_order)
_
Patches currently in -mm which might be from david@xxxxxxxxxx are
mm-madvise-dont-perform-madvise-vma-walk-for-madv_populate_readwrite.patch
mm-convert-folio_estimated_sharers-to-folio_likely_mapped_shared.patch
mm-convert-folio_estimated_sharers-to-folio_likely_mapped_shared-fix.patch
selftests-memfd_secret-add-vmsplice-test.patch
mm-merge-folio_is_secretmem-and-folio_fast_pin_allowed-into-gup_fast_folio_allowed.patch
mm-optimize-config_per_vma_lock-member-placement-in-vm_area_struct.patch
mm-remove-prot-parameter-from-move_pte.patch
mm-gup-consistently-name-gup-fast-functions.patch
mm-treewide-rename-config_have_fast_gup-to-config_have_gup_fast.patch
mm-use-gup-fast-instead-fast-gup-in-remaining-comments.patch
drivers-virt-acrn-fix-pfnmap-pte-checks-in-acrn_vm_ram_map.patch
mm-pass-vma-instead-of-mm-to-follow_pte.patch
mm-follow_pte-improvements.patch
mm-allow-for-detecting-underflows-with-page_mapcount-again.patch
mm-rmap-always-inline-anon-file-rmap-duplication-of-a-single-pte.patch
mm-rmap-add-fast-path-for-small-folios-when-adding-removing-duplicating.patch
mm-track-mapcount-of-large-folios-in-single-value.patch
mm-improve-folio_likely_mapped_shared-using-the-mapcount-of-large-folios.patch
mm-make-folio_mapcount-return-0-for-small-typed-folios.patch
mm-memory-use-folio_mapcount-in-zap_present_folio_ptes.patch
mm-huge_memory-use-folio_mapcount-in-zap_huge_pmd-sanity-check.patch
mm-memory-failure-use-folio_mapcount-in-hwpoison_user_mappings.patch
mm-page_alloc-use-folio_mapped-in-__alloc_contig_migrate_range.patch
mm-migrate-use-folio_likely_mapped_shared-in-add_page_for_migration.patch
sh-mm-cache-use-folio_mapped-in-copy_from_user_page.patch
mm-filemap-use-folio_mapcount-in-filemap_unaccount_folio.patch
mm-migrate_device-use-folio_mapcount-in-migrate_vma_check_page.patch
trace-events-page_ref-trace-the-raw-page-mapcount-value.patch
xtensa-mm-convert-check_tlb_entry-to-sanity-check-folios.patch
mm-debug-print-only-page-mapcount-excluding-folio-entire-mapcount-in-__dump_folio.patch
documentation-admin-guide-cgroup-v1-memoryrst-dont-reference-page_mapcount.patch
mm-ksm-rename-get_ksm_page_flags-to-ksm_get_folio_flags.patch
mm-ksm-remove-page_mapcount-usage-in-stable_tree_search.patch
fs-proc-task_mmu-convert-pagemap_hugetlb_range-to-work-on-folios.patch
fs-proc-task_mmu-convert-smaps_hugetlb_range-to-work-on-folios.patch
mm-huge_memory-improve-split_huge_page_to_list_to_order-return-value-documentation.patch
loongarch-tlb-fix-error-parameter-ptep-set-but-not-used-due-to-__tlb_remove_tlb_entry.patch
