Minki/linux
1
0
Fork 1
mirror of https://github.com/torvalds/linux.git synced 2024-11-22 12:11:40 +00:00
Code Issues Packages Projects Releases Wiki Activity

huge_memory.c: document huge page splitting rules more thoroughly

Browse Source 
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: John Hubbard <jhubbard@nvidia.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
John Hubbard 2024-03-24 21:44:52 -07:00 committed by Andrew Morton
parent d4e6b397be
commit a8353dc98f
1 changed files with 27 additions and 15 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

42
mm/huge_memory.c
View File

@ -3006,28 +3006,40 @@ 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,
* since they do not use _deferred_list.
* Prerequisites:
*
* Only caller must hold pin on the @page, otherwise split fails with -EBUSY.
* The huge page must be locked.
* 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.
*
* 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)