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@xxxxxxxxxxxxx>
---
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
