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. Cc: David Hildenbrand <david@xxxxxxxxxx> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx> Cc: Zi Yan <ziy@xxxxxxxxxx> Signed-off-by: John Hubbard <jhubbard@xxxxxxxxxx> --- Hi David, Matthew, Zi, This is a follow up from our short email thread of a week ago [1]. Zi, I've inflicted some minor violence upon your original wording, and moved it into a Prerequisites section (item 4). [1] https://lore.kernel.org/all/d9c06bec-805f-4d53-9f91-6b8ad29fcb6b@xxxxxxxxxx/ thanks, John Hubbard NVIDIA mm/huge_memory.c | 42 +++++++++++++++++++++++++++--------------- 1 file changed, 27 insertions(+), 15 deletions(-) diff --git a/mm/huge_memory.c b/mm/huge_memory.c index 9859aa4f7553..9f2354068359 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -3013,28 +3013,40 @@ 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) -- 2.44.0
