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mm/gup: Handle page split race more efficiently

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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>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: William Kucharski <william.kucharski@oracle.com>
This commit is contained in:
Matthew Wilcox (Oracle) 2022-01-07 14:04:55 -05:00
parent 4c65422901
commit 59409373f6
1 changed files with 5 additions and 2 deletions
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7
mm/gup.c
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@ -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;