[5.4 PATCH] mm/gup: Do not force a COW break on file-backed memory

["Matthew Wilcox (Oracle)" <willy@infradead.org>]

Commit 17839856fd58 ("gup: document and work around "COW can break either
way" issue") forces a COW break, even for read-only GUP.  This interacts
badly with CONFIG_READ_ONLY_THP_FOR_FS as it tries to write to a read-only
PMD and follow_trans_huge_pmd() returns NULL which induces an endless
loop as __get_user_pages() interprets that as page-not-present, tries
to fault it in and retries the follow_page_mask().

The issues fixed by 17839856fd58 don't apply to files.  We know which way
the COW breaks; the page cache keeps the original and any modifications
are private to that process.  There's no optimisation that allows a
process to reuse a file-backed MAP_PRIVATE page.  So we can skip the
breaking of the COW for file-backed mappings.

This problem only exists in v5.4.y; other stable kernels either predate
CONFIG_READ_ONLY_THP_FOR_FS or they include commit a308c71bf1e6 ("mm/gup:
Remove enfornced COW mechanism").

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
---
 mm/gup.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/gup.c b/mm/gup.c
index 3ef769529548..d55e02411010 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -176,7 +176,8 @@ static inline bool can_follow_write_pte(pte_t pte, unsigned int flags)
  */
 static inline bool should_force_cow_break(struct vm_area_struct *vma, unsigned int flags)
 {
-	return is_cow_mapping(vma->vm_flags) && (flags & FOLL_GET);
+	return is_cow_mapping(vma->vm_flags) && vma_is_anonymous(vma) &&
+		(flags & FOLL_GET);
 }
 
 static struct page *follow_page_pte(struct vm_area_struct *vma,
-- 
2.33.0