mm/page_alloc: warn on nr_reserved_highatomic underflow

As documented in the comment this underflow should not happen.  The
locking has indeed changed here since the comment was written, see the
migratetype hygiene patches[0].  However, those changes made the locking
_safer_, so the underflow _really_ shouldn't happen now.  So upgrade the
comment to a warning.

[0] https://lore.kernel.org/all/20240320180429.678181-7-hannes@cmpxchg.org/T/#m3da87e6cc3348a4640aa298137bc9f8f61b76c84

Link: https://lkml.kernel.org/r/20250225-warn-underflow-v1-1-3dc542941d3a@google.com
Signed-off-by: Brendan Jackman <jackmanb@google.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index c4fedbe7f9add207e41757557e466ffe25bcf730..eab307690db8f8e682e015acbf225b78885c705b 100644 (file)
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3095,6 +3095,7 @@ static bool unreserve_highatomic_pageblock(const struct alloc_context *ac,
                        if (!page)
                                continue;
 
+                       size = max(pageblock_nr_pages, 1UL << order);
                        /*
                         * It should never happen but changes to
                         * locking could inadvertently allow a per-cpu
@@ -3102,8 +3103,8 @@ static bool unreserve_highatomic_pageblock(const struct alloc_context *ac,
                         * while unreserving so be safe and watch for
                         * underflows.
                         */
-                       size = max(pageblock_nr_pages, 1UL << order);
-                       size = min(size, zone->nr_reserved_highatomic);
+                       if (WARN_ON_ONCE(size > zone->nr_reserved_highatomic))
+                               size = zone->nr_reserved_highatomic;
                        zone->nr_reserved_highatomic -= size;
 
                        /*