Re: [PATCH v2] mm: migrate: Fix THP's mapcount on isolation

From: Gavin Shan
Date: Thu Nov 24 2022 - 05:23:23 EST


On 11/24/22 6:09 PM, David Hildenbrand wrote:
On 24.11.22 10:55, Gavin Shan wrote:
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.

Did you look into handling pages that are in the swapcache case as well?

See is_refcount_suitable() in mm/khugepaged.c.

Should be easy to reproduce, let me know if you need inspiration.


Nope, I didn't look into the case. Please elaborate the details so that
I can reproduce it firstly.


Fixes: 1da2f328fa64 ("mm,thp,compaction,cma: allow THP migration for CMA allocations")
Cc: stable@xxxxxxxxxxxxxxx   # v5.7+
Reported-by: Zhenyu Zhang <zhenyzha@xxxxxxxxxx>
Suggested-by: David Hildenbrand <david@xxxxxxxxxx>
Signed-off-by: Gavin Shan <gshan@xxxxxxxxxx>
---
v2: Corrected fix tag and increase page's refcount before the check
---
  mm/compaction.c | 22 +++++++++++-----------
  1 file changed, 11 insertions(+), 11 deletions(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index c51f7f545afe..1f6da31dd9a5 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -984,29 +984,29 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
              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;
+
          /*
           * 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))


Thanks,
Gavin