[Xen-devel] [XEN PATCH] Check zone before merging adjacent blocks in heap

[Stewart Hildebrand <stewart.hildebrand@xxxxxxxxxxxxxxx>]

From: Jeff Kubascik <jeff.kubascik@xxxxxxxxxxxxxxx>

The Xen heap is split up into nodes and zones. Each node + zone is
managed as a separate pool of memory.

When returning pages to the heap, free_heap_pages will check adjacent
blocks to see if they can be combined into a larger block. However, the
zone of the adjacent block is not checked. This results in blocks that
migrate from one zone to another.

When a block migrates to the adjacent zone, the avail counters for the
old and new node + zone is not updated accordingly. The avail counter
is used when allocating pages to determine whether to skip over a zone.
With this behavior, it is possible for free pages to collect in a zone
with the avail counter smaller than the actual page count, resulting
in free pages that are not allocable.

This commit adds a check to compare the adjacent block's zone with the
current zone before merging them.

Signed-off-by: Jeff Kubascik <jeff.kubascik@xxxxxxxxxxxxxxx>
---

Since this topic came up again, I figure it makes sense to resend it as
a real patch using git send-email rather than in reply to an existing
email.

---
 xen/common/page_alloc.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/xen/common/page_alloc.c b/xen/common/page_alloc.c
index 97902d42c1..7d39dd5be0 100644
--- a/xen/common/page_alloc.c
+++ b/xen/common/page_alloc.c
@@ -1462,6 +1462,7 @@ static void free_heap_pages(
             if ( !mfn_valid(page_to_mfn(predecessor)) ||
                  !page_state_is(predecessor, free) ||
                  (PFN_ORDER(predecessor) != order) ||
+                 (page_to_zone(pg-mask) != zone) ||
                  (phys_to_nid(page_to_maddr(predecessor)) != node) )
                 break;
 
@@ -1485,6 +1486,7 @@ static void free_heap_pages(
             if ( !mfn_valid(page_to_mfn(successor)) ||
                  !page_state_is(successor, free) ||
                  (PFN_ORDER(successor) != order) ||
+                 (page_to_zone(pg+mask) != zone) ||
                  (phys_to_nid(page_to_maddr(successor)) != node) )
                 break;
 
-- 
2.25.0


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