On 1/9/22 20:23, Matthew Wilcox (Oracle) wrote:
If we hit the page split race, the current code returns NULL which will presumably trigger a retry under the mmap_lock. This isn't necessary; we can just retry the compound_head() lookup. This is a very minor optimisation of an unlikely path, but conceptually it matches (eg) the page cache RCU-protected lookup. Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx> --- mm/gup.c | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-)
Reviewed-by: John Hubbard <jhubbard@xxxxxxxxxx> thanks, -- John Hubbard NVIDIA
diff --git a/mm/gup.c b/mm/gup.c index afb638a30e44..dbb1b54d0def 100644 --- a/mm/gup.c +++ b/mm/gup.c @@ -68,7 +68,10 @@ static void put_page_refs(struct page *page, int refs) */ static inline struct page *try_get_compound_head(struct page *page, int refs) { - struct page *head = compound_head(page); + struct page *head; + +retry: + head = compound_head(page);if (WARN_ON_ONCE(page_ref_count(head) < 0))return NULL; @@ -86,7 +89,7 @@ static inline struct page *try_get_compound_head(struct page *page, int refs) */ if (unlikely(compound_head(page) != head)) { put_page_refs(head, refs); - return NULL; + goto retry; }return head;
