Commit d21f996b authored by David Hildenbrand's avatar David Hildenbrand Committed by Andrew Morton
Browse files

mm/huge_memory: improve split_huge_page_to_list_to_order() return value documentation 

The documentation is wrong and relying on it almost resulted in BUGs in
new callers: ever since fd4a7ac3 ("mm: migrate: try again if THP split
is failed due to page refcnt") 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.

[david@redhat.com: v2]
  Link: https://lkml.kernel.org/r/20240422194217.442933-1-david@redhat.com
Link: https://lkml.kernel.org/r/20240418151834.216557-1-david@redhat.com


Signed-off-by: default avatarDavid Hildenbrand <david@redhat.com>
Reviewed-by: default avatarZi Yan <ziy@nvidia.com>
Reviewed-by: default avatarJohn Hubbard <jhubbard@nvidia.com>
Reviewed-by: default avatarBaolin Wang <baolin.wang@linux.alibaba.com>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
parent 8430557f

mm/huge_memory.c

+11 −3
Original line number Diff line number Diff line
@@ -2956,7 +2956,7 @@ bool can_split_folio(struct folio *folio, int *pextra_pins) 
 *

 * 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 +2975,16 @@ bool can_split_folio(struct folio *folio, int *pextra_pins) 
 *

 * 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) or if

 * the folio was concurrently removed from the page cache.

 *

 * 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)