| Message ID | 20220110042406.499429-9-willy@infradead.org (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | Convert GUP to folios | expand |
On Mon, Jan 10, 2022 at 04:23:46AM +0000, 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@infradead.org> Looks good, Reviewed-by: Christoph Hellwig <hch@lst.de>
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@infradead.org> > --- > mm/gup.c | 7 +++++-- > 1 file changed, 5 insertions(+), 2 deletions(-) Reviewed-by: John Hubbard <jhubbard@nvidia.com> thanks,
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;
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@infradead.org> --- mm/gup.c | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-)