mm: migrate: fix THP's mapcount on isolation

The issue is reported when removing memory through virtio_mem device.  The
transparent huge page, experienced copy-on-write fault, is wrongly
regarded as pinned.  The transparent huge page is escaped from being
isolated in isolate_migratepages_block().  The transparent huge page can't
be migrated and the corresponding memory block can't be put into offline
state.

Fix it by replacing page_mapcount() with total_mapcount().  With this, the
transparent huge page can be isolated and migrated, and the memory block
can be put into offline state.  Besides, The page's refcount is increased
a bit earlier to avoid the page is released when the check is executed.

Link: https://lkml.kernel.org/r/20221124095523.31061-1-gshan@redhat.com
Fixes: 1da2f328fa64 ("mm,thp,compaction,cma: allow THP migration for CMA allocations")
Signed-off-by: Gavin Shan <gshan@redhat.com>
Reported-by: Zhenyu Zhang <zhenyzha@redhat.com>
Tested-by: Zhenyu Zhang <zhenyzha@redhat.com>
Suggested-by: David Hildenbrand <david@redhat.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: William Kucharski <william.kucharski@oracle.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: <stable@vger.kernel.org>	[5.7+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/compaction.c b/mm/compaction.c
index c51f7f5..1f6da31 100644 (file)
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -985,28 +985,28 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
                }
 
                /*
+                * Be careful not to clear PageLRU until after we're
+                * sure the page is not being freed elsewhere -- the
+                * page release code relies on it.
+                */
+               if (unlikely(!get_page_unless_zero(page)))
+                       goto isolate_fail;
+
+               /*
                 * Migration will fail if an anonymous page is pinned in memory,
                 * so avoid taking lru_lock and isolating it unnecessarily in an
                 * admittedly racy check.
                 */
                mapping = page_mapping(page);
-               if (!mapping && page_count(page) > page_mapcount(page))
-                       goto isolate_fail;
+               if (!mapping && (page_count(page) - 1) > total_mapcount(page))
+                       goto isolate_fail_put;
 
                /*
                 * Only allow to migrate anonymous pages in GFP_NOFS context
                 * because those do not depend on fs locks.
                 */
                if (!(cc->gfp_mask & __GFP_FS) && mapping)
-                       goto isolate_fail;
-
-               /*
-                * Be careful not to clear PageLRU until after we're
-                * sure the page is not being freed elsewhere -- the
-                * page release code relies on it.
-                */
-               if (unlikely(!get_page_unless_zero(page)))
-                       goto isolate_fail;
+                       goto isolate_fail_put;
 
                /* Only take pages on LRU: a check now makes later tests safe */
                if (!PageLRU(page))