Commit 8e0dbe05 authored by John Hubbard's avatar John Hubbard Committed by Liu Shixin
Browse files

huge_memory.c: document huge page splitting rules more thoroughly 

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category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I9OCYO
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=a8353dc98f3ae570297e5e25cc05fc7d6b7f0e7b

--------------------------------

1. Add information about the behavior of huge page splitting, with
   respect to page/folio refcounts, and gup/pup pins.

2. Update and clarify the existing documentation, to compensate for the
   ravages of time and code change.

Link: https://lkml.kernel.org/r/20240325044452.217463-1-jhubbard@nvidia.com


Signed-off-by: default avatarJohn Hubbard <jhubbard@nvidia.com>
Reviewed-by: default avatarZi Yan <ziy@nvidia.com>
Reviewed-by: default avatarDavid Hildenbrand <david@redhat.com>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLiu Shixin <liushixin2@huawei.com>
parent b8eba2d7

mm/huge_memory.c

+27 −15
Original line number Diff line number Diff line
@@ -2879,28 +2879,40 @@ bool can_split_folio(struct folio *folio, int *pextra_pins) 
}



/*

 * This function splits huge page into pages in @new_order. @page can point to

 * any subpage of huge page to split. Split doesn't change the position of

 * @page.

 * This function splits a large folio into smaller folios of order @new_order.

 * @page can point to any page of the large folio to split. The split operation

 * does not change the position of @page.

 *

 * NOTE: order-1 anonymous folio is not supported because _deferred_list,

 * which is used by partially mapped folios, is stored in subpage 2 and an

 * order-1 folio only has subpage 0 and 1. File-backed order-1 folios are OK,

 * Prerequisites:

 *

 * 1) The caller must hold a reference on the @page's owning folio, also known

 *    as the large folio.

 *

 * 2) The large folio must be locked.

 *

 * 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.

 *

 * 4) @new_order > 1, usually. Splitting to order-1 anonymous folios is not

 *    supported for non-file-backed folios, because folio->_deferred_list, which

 *    is used by partially mapped folios, is stored in subpage 2, but an order-1

 *    folio only has subpages 0 and 1. File-backed order-1 folios are supported,

 *    since they do not use _deferred_list.

 *

 * Only caller must hold pin on the @page, otherwise split fails with -EBUSY.

 * The huge page must be locked.

 * After splitting, the caller's folio reference will be transferred to @page,

 * resulting in a raised refcount of @page after this call. The other pages may

 * be freed if they are not mapped.

 *

 * If @list is null, tail pages will be added to LRU list, otherwise, to @list.

 *

 * Pages in new_order will inherit mapping, flags, and so on from the hugepage.

 * Pages in @new_order will inherit the mapping, flags, and so on from the

 * huge page.

 *

 * GUP pin and PG_locked transferred to @page or the compound page @page belongs

 * to. Rest subpages can be freed if they are not mapped.

 * Returns 0 if the huge page was split successfully.

 *

 * Returns 0 if the hugepage is split successfully.

 * Returns -EBUSY if the page is pinned or if anon_vma disappeared from under

 * us.

 * Returns -EBUSY if @page's folio is pinned, or if the anon_vma disappeared

 * from under us.

 */

int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,

				     unsigned int new_order)