mm/gup: check every subpage of a compound page during isolation

When pages are isolated in check_and_migrate_movable_pages() we skip
compound number of pages at a time.  However, as Jason noted, it is not
necessary correct that pages[i] corresponds to the pages that we
skipped.  This is because it is possible that the addresses in this
range had split_huge_pmd()/split_huge_pud(), and these functions do not
update the compound page metadata.

The problem can be reproduced if something like this occurs:

1. User faulted huge pages.
2. split_huge_pmd() was called for some reason
3. User has unmapped some sub-pages in the range
4. User tries to longterm pin the addresses.

The resulting pages[i] might end-up having pages which are not compound
size page aligned.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: aa712399c1e8 ("mm/gup: speed up check_and_migrate_cma_pages() on huge page")
Signed-off-by: Pavel Tatashin <[email protected]>
Reported-by: Jason Gunthorpe <[email protected]>
Reviewed-by: Jason Gunthorpe <[email protected]>
Cc: Dan Williams <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Ira Weiny <[email protected]>
Cc: James Morris <[email protected]>
Cc: Jason Gunthorpe <[email protected]>
Cc: John Hubbard <[email protected]>
Cc: Joonsoo Kim <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Mike Kravetz <[email protected]>
Cc: Oscar Salvador <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Sasha Levin <[email protected]>
Cc: Steven Rostedt (VMware) <[email protected]>
Cc: Tyler Hicks <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

1 files changed, 7 insertions, 12 deletions

diff --git a/mm/gup.c b/mm/gup.c
index b63e2c0df3c7..16015718c0df 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1609,26 +1609,23 @@ static long check_and_migrate_cma_pages(struct mm_struct *mm,
 					unsigned int gup_flags)
 {
 	unsigned long i;
-	unsigned long step;
 	bool drain_allow = true;
 	bool migrate_allow = true;
 	LIST_HEAD(cma_page_list);
 	long ret = nr_pages;
+	struct page *prev_head, *head;
 	struct migration_target_control mtc = {
 		.nid = NUMA_NO_NODE,
 		.gfp_mask = GFP_USER | __GFP_NOWARN,
 	};
 
 check_again:
-	for (i = 0; i < nr_pages;) {
-
-		struct page *head = compound_head(pages[i]);
-
-		/*
-		 * gup may start from a tail page. Advance step by the left
-		 * part.
-		 */
-		step = compound_nr(head) - (pages[i] - head);
+	prev_head = NULL;
+	for (i = 0; i < nr_pages; i++) {
+		head = compound_head(pages[i]);
+		if (head == prev_head)
+			continue;
+		prev_head = head;
 		/*
 		 * If we get a page from the CMA zone, since we are going to
 		 * be pinning these entries, we might as well move them out
@@ -1652,8 +1649,6 @@ check_again:
 				}
 			}
 		}
-
-		i += step;
 	}
 
 	if (!list_empty(&cma_page_list)) {