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[Xen-devel] [PATCH] mm/page_alloc: correct first_dirty calculations during block merging

Currently it's possible to hit an assertion in alloc_heap_pages():

Assertion 'first_dirty != INVALID_DIRTY_IDX || !(pg[i].count_info & 
PGC_need_scrub)' failed at page_alloc.c:988

This can happen because a piece of logic to calculate first_dirty
during block merging in free_heap_pages() is missing for the following

1. Current block's first_dirty equals to INVALID_DIRTY_IDX
2. Successor block is free but its first_dirty != INVALID_DIRTY_IDX
3. The successor is merged into current block
4. Current block's first_dirty still equals to INVALID_DIRTY_IDX

This will trigger the assertion during allocation of such block in
alloc_heap_pages() because there will be pages with PGC_need_scrub
bit set despite the claim of first_dirty that the block is scrubbed.

Add the missing piece of logic and slightly update the comment for
the predecessor case to better capture the code's intent.

Fixes 1a37f33ea613 ("mm: Place unscrubbed pages at the end of pagelist")

Signed-off-by: Sergey Dyasli <sergey.dyasli@xxxxxxxxxx>
CC: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CC: George Dunlap <George.Dunlap@xxxxxxxxxxxxx>
CC: Jan Beulich <jbeulich@xxxxxxxx>
CC: Julien Grall <julien.grall@xxxxxxx>
CC: Wei Liu <wei.liu2@xxxxxxxxxx>
CC: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
 xen/common/page_alloc.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/xen/common/page_alloc.c b/xen/common/page_alloc.c
index 20ee1e4897..aa911f2dc5 100644
--- a/xen/common/page_alloc.c
+++ b/xen/common/page_alloc.c
@@ -1426,7 +1426,7 @@ static void free_heap_pages(
             page_list_del(predecessor, &heap(node, zone, order));
-            /* Keep predecessor's first_dirty if it is already set. */
+            /* Keep block's first_dirty if the predecessor doesn't have one */
             if ( predecessor->u.free.first_dirty == INVALID_DIRTY_IDX &&
                  pg->u.free.first_dirty != INVALID_DIRTY_IDX )
                 predecessor->u.free.first_dirty = (1U << order) +
@@ -1447,6 +1447,12 @@ static void free_heap_pages(
+            /* Keep successor's first_dirty if the block doesn't have one */
+            if ( pg->u.free.first_dirty == INVALID_DIRTY_IDX &&
+                 successor->u.free.first_dirty != INVALID_DIRTY_IDX )
+                pg->u.free.first_dirty = (1U << order) +
+                                         successor->u.free.first_dirty;
             page_list_del(successor, &heap(node, zone, order));

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