mm/page_alloc: count guard pages as free again when their buddy merges

With debug_guardpage_minorder set, expand() turns the unused halves of a
split into guard pages, which page_del_and_expand() leaves out of the free
counts.  Freeing the allocated half credits only that half, and
__free_one_page() then merges the guard buddy with clear_page_guard(),
which no longer touches the counters.

The merged block goes on a free list with the guard pages never counted,
so NR_FREE_PAGES falls behind the free lists by every guard that merges. 
Before commit e0932b6c1f94 ("mm: page_alloc: consolidate free page
accounting"), __set_page_guard() and __clear_page_guard() adjusted the
counts themselves; that commit dropped both adjustments but kept the
subtraction implicit in expand().

Count the guard pages under the merged block's migratetype when the buddy
is cleared.  account_freepages() skips an isolated block as it does for
every other free page, and moving the block off the isolated list counts
it then.

Booting a 16GB VM with debug_pagealloc=on debug_guardpage_minorder=1,
check the difference in free pages reported between /proc/vmstat and
/proc/buddyinfo.

At three points (idle, after a read and file-creation load, after a second
load), check the difference between nr_free_pages and the buddyinfo
numbers in the normal zone, as a number of pages:

                idle   after load   after second load
    unpatched   -130        79205               99121
    patched      -16           45                   0

The small remaining difference seems to be due to the numbers not being
read at exactly the same time, and is also seen without guard pages.

Link: https://lore.kernel.org/20261016122304.5b1eba8e@fangorn
Fixes: e0932b6c1f94 ("mm: page_alloc: consolidate free page accounting")
Signed-off-by: Rik van Riel <riel@surriel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Dev Jain <dev.jain@arm.com>
Reviewed-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
Assisted-by: LLM
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Brendan Jackman <brendan.jackman@linux.dev>
Cc: Kairui Song <kasong@tencent.com>
Cc: Qi Zheng <qi.zheng@linux.dev>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Baoquan He <baoquan.he@linux.dev>
Cc: David Hildenbrand <david@kernel.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Frank van der Linden <fvdl@google.com>
Cc: <stable@vger.kernel.org>

0 files changed, 0 insertions, 0 deletions