From 607c63195d6388c4b70bea104708221392564893 Mon Sep 17 00:00:00 2001 From: Peter Xu Date: Wed, 27 Mar 2024 11:23:25 -0400 Subject: [PATCH] mm/gup: drop gup_fast_folio_allowed() in hugepd processing Hugepd format for GUP is only used in PowerPC with hugetlbfs. There are some kernel usage of hugepd (can refer to hugepd_populate_kernel() for PPC_8XX), however those pages are not candidates for GUP. Commit a6e79df92e4a ("mm/gup: disallow FOLL_LONGTERM GUP-fast writing to file-backed mappings") added a check to fail gup-fast if there's potential risk of violating GUP over writeback file systems. That should never apply to hugepd. Considering that hugepd is an old format (and even software-only), there's no plan to extend hugepd into other file typed memories that is prone to the same issue. Drop that check, not only because it'll never be true for hugepd per any known plan, but also it paves way for reusing the function outside fast-gup. To make sure we'll still remember this issue just in case hugepd will be extended to support non-hugetlbfs memories, add a rich comment above gup_huge_pd(), explaining the issue with proper references. [akpm@linux-foundation.org: fix comment, per David] Link: https://lkml.kernel.org/r/20240327152332.950956-7-peterx@redhat.com Signed-off-by: Peter Xu Tested-by: Ryan Roberts Cc: Christoph Hellwig Cc: Lorenzo Stoakes Cc: Michael Ellerman Cc: Andrea Arcangeli Cc: Andrew Jones Cc: Aneesh Kumar K.V (IBM) Cc: Axel Rasmussen Cc: Christophe Leroy Cc: David Hildenbrand Cc: James Houghton Cc: Jason Gunthorpe Cc: John Hubbard Cc: Kirill A. Shutemov Cc: Matthew Wilcox (Oracle) Cc: "Mike Rapoport (IBM)" Cc: Muchun Song Cc: Rik van Riel Cc: Vlastimil Babka Cc: Yang Shi Signed-off-by: Andrew Morton --- mm/gup.c | 13 ++++++++----- 1 file changed, 8 insertions(+), 5 deletions(-) diff --git a/mm/gup.c b/mm/gup.c index 7b21fcda3a98..1cc3240e4900 100644 --- a/mm/gup.c +++ b/mm/gup.c @@ -2842,11 +2842,6 @@ static int gup_hugepte(pte_t *ptep, unsigned long sz, unsigned long addr, return 0; } - if (!gup_fast_folio_allowed(folio, flags)) { - gup_put_folio(folio, refs, flags); - return 0; - } - if (!pte_write(pte) && gup_must_unshare(NULL, flags, &folio->page)) { gup_put_folio(folio, refs, flags); return 0; @@ -2857,6 +2852,14 @@ static int gup_hugepte(pte_t *ptep, unsigned long sz, unsigned long addr, return 1; } +/* + * NOTE: currently GUP for a hugepd is only possible on hugetlbfs file + * systems on Power, which does not have issue with folio writeback against + * GUP updates. When hugepd will be extended to support non-hugetlbfs or + * even anonymous memory, we need to do extra check as what we do with most + * of the other folios. See writable_file_mapping_allowed() and + * gup_fast_folio_allowed() for more information. + */ static int gup_huge_pd(hugepd_t hugepd, unsigned long addr, unsigned int pdshift, unsigned long end, unsigned int flags, struct page **pages, int *nr)