[PATCH 5.12 221/363] mm/gup: check every subpage of a compound page during isolation

[Greg Kroah-Hartman]

From: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>

[ Upstream commit 83c02c23d0747a7bdcd71f99a538aacec94b146c ]

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/20210215161349.246722-3-pasha.tatashin@xxxxxxxxxx
Fixes: aa712399c1e8 ("mm/gup: speed up check_and_migrate_cma_pages() on huge page")
Signed-off-by: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>
Reported-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
Reviewed-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
Cc: David Hildenbrand <david@xxxxxxxxxx>
Cc: David Rientjes <rientjes@xxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Ira Weiny <ira.weiny@xxxxxxxxx>
Cc: James Morris <jmorris@xxxxxxxxx>
Cc: Jason Gunthorpe <jgg@xxxxxxxx>
Cc: John Hubbard <jhubbard@xxxxxxxxxx>
Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
Cc: Oscar Salvador <osalvador@xxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Sasha Levin <sashal@xxxxxxxxxx>
Cc: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
Cc: Tyler Hicks <tyhicks@xxxxxxxxxxxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 mm/gup.c | 19 +++++++------------
 1 file changed, 7 insertions(+), 12 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index ef7d2da9f03f..6530672131db 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1552,26 +1552,23 @@ 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_MOVABLE | __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
@@ -1595,8 +1592,6 @@ check_again:
 				}
 			}
 		}
-
-		i += step;
 	}
 
 	if (!list_empty(&cma_page_list)) {
-- 
2.30.2