[PATCH 0/2] xen/mm: Fix off-by-one for tail merge in reserve_offlined_page()

[Bernhard Kaindl <bernhard.kaindl@xxxxxxxxxx>]

After offlining pages, reserve_offlined_page() returns healthy
spans of pages from the buddy it isolated the offlined pages
back to the free lists.

Naturally, it attempts to grow larger buddies, but due to an
off-by-one, this fails at the tail end of the span of pages.

Patch 1 adds a native regression test that reproduces the problem.
Patch 2 fixes the off-by-one error and updates the regression test.

Consider an order-2 buddy (4 pages) with the following layout:
+---------------+---------------+---------------+---------------+
| head page       tail page 1,    tail page 2     tail page 3   |
| PFN_ORDER(pg)   marked as to                                  |
| == 2            be offlined                                   |
+---------------+---------------+---------------+---------------+

The expected result after removing tail page 1 and returning the
remaining healthy pages to the free list would be:

+---------------+               +---------------+---------------+
| single page   | offlined page | head page       tail page     |
| PFN_ORDER(pg) | not returned  | PFN_ORDER(pg)                 |
| == 0          | to the heap   | == 1                          |
+---------------+               +---------------+---------------+

A trivial off-by-one error in the growth loop stops the growth loop
early before the tail end of the original buddy and we end up with:

+---------------+               +---------------+---------------+
| single page   | offlined page | single page   | single page   |
| PFN_ORDER(pg) | not returned  | PFN_ORDER(pg) | PFN_ORDER(pg) |
| == 0          | to the heap   | == 0          | == 0          |
+---------------+               +---------------+---------------+

This series is based on the native test environment v3 for NUMA claims
https://lists.xen.org/archives/html/xen-devel/2026-05/msg01163.html

In turn, it is based on the NUMA claim sets v7 series:
https://lists.xen.org/archives/html/xen-devel/2026-05/msg00363.html

You can pull this series with all dependencies for review and test:
$ git pull git@xxxxxxxxxx:bernhardkaindl/xen.git offline-merge-tail-v1
$ make -C tools/tests/native TARGETS=offline-merge-tail test

Fixes: e4865c2315 ('Page offline support in Xen side')
Signed-off-by: Bernhard Kaindl <bernhard.kaindl@xxxxxxxxxx>

Bernhard Kaindl (2):
  tools/tests/native: Test merging the tail after an offlined page
  xen/mm: Fix off-by-one stopping tail merge in reserve_offlined_page

 tools/tests/native/offline-merge-tail.c | 87 +++++++++++++++++++++++++
 xen/common/page_alloc.c                 |  4 +-
 2 files changed, 90 insertions(+), 1 deletion(-)
 create mode 100644 tools/tests/native/offline-merge-tail.c

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2.39.5