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b=awZxtyZwecddB6EvRPOgcqqGvA IRcXu9Yscs0Joo/AXlv0RwllD4aKOn6TlWwsD/aTzEmiIoXbc7A7xCBtrOWj1BEMaBHN1NGM+/HPq bJJJ0pic+Vg4BF/wVt/hP5TIwMHmMxqQiWXA/mYA4v+jp11ld7aB2fPQlS8uh1rowrzXCFOov9xs+ hro+ThRizVBODDqPqs0LeLxC9Mg8YdGIZgdYALhYVtuev7iyUNJKqWTU7QW0jAxFeOCOXSgWe92ln lm2mkRVbijdou3SwLRLQRXiA5nTG0Iv5GHEjtB6Kg/txhs+X5n82zQYvRZraewN+P2umAec/UF5zn 3MFqto3Q==; Received: from fangorn.home.surriel.com ([10.0.13.7]) by shelob.surriel.com with esmtpsa (TLS1.2) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.97.1) (envelope-from ) id 1wIXuC-000000001R0-45zn; Thu, 30 Apr 2026 16:22:40 -0400 From: Rik van Riel To: linux-kernel@vger.kernel.org Cc: kernel-team@meta.com, linux-mm@kvack.org, david@kernel.org, willy@infradead.org, surenb@google.com, hannes@cmpxchg.org, ljs@kernel.org, ziy@nvidia.com, usama.arif@linux.dev, Rik van Riel , Rik van Riel Subject: [RFC PATCH 23/45] mm: page_alloc: add CONFIG_DEBUG_VM sanity checks for SPB counters Date: Thu, 30 Apr 2026 16:20:52 -0400 Message-ID: <20260430202233.111010-24-riel@surriel.com> X-Mailer: git-send-email 2.52.0 In-Reply-To: <20260430202233.111010-1-riel@surriel.com> References: <20260430202233.111010-1-riel@surriel.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Stat-Signature: zm8c3g7nh878wprichdsa93jco68fhmc X-Rspam-User: X-Rspamd-Queue-Id: B320840008 X-Rspamd-Server: rspam07 X-HE-Tag: 1777580580-377645 X-HE-Meta: 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 YehwTJ/w tAIzXRNr3OL88n1WFlqqrQ6PdSDtEQQeTM8CM8kQaBZgwrMGSdLUJq0Uo5EexKFRVxpycqJmU8k6Pywx9E+ipovRL+neGOD1lhqhg5ziFTreffI9HS0g52lgzWN61QIrddNWaHSanaTThqRO/mK9U08tRUQD/HjEJS/DRN91jZXnEmaoDo5p28iIpDIYmcoMFOdWFE3cwtcXkWBrN4Y3wF5hwBLYPyIEZ0rD7cHhNFjv4mVzGDdAuuwMDLjP1v7F1QHTNCxR1iCSZBqifke3VqiyrsSzI/ElIwyXTp5mJyWATpLmiu6dg6GLFTSK5b0YBDluV+oRwkYSPpqrRNWzSlVKVJo2ZAj971jXjenCA0yHlIG8nSYs4gsLaS6t1VpwazzVx Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: From: Rik van Riel Add spb_debug_check() and call it after every site that mutates the per-superpageblock type counters (nr_free / nr_unmovable / nr_reclaimable / nr_movable). Each counter must be <= total_pageblocks; a violation indicates that a PB_has_ bit transition was missed by one of the allocation, free, claim, or evacuation paths and the counter has drifted out of sync with the bits. VM_WARN_ONCE keeps the production cost zero (CONFIG_DEBUG_VM only) while giving us a single place to catch counter drift early during stress testing instead of debugging it from a much later misaccounting symptom. Several real bugs in the SPB stack were caught by this check during development; keeping it permanently makes future churn safer. Signed-off-by: Rik van Riel Assisted-by: Claude:claude-opus-4.7 syzkaller --- mm/page_alloc.c | 46 +++++++++++++++++++++++++++++++++++++++++----- 1 file changed, 41 insertions(+), 5 deletions(-) diff --git a/mm/page_alloc.c b/mm/page_alloc.c index 4f8105b89e47..9f4298fc2727 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -521,6 +521,32 @@ static inline int migratetype_to_has_bit(int migratetype) } } +#ifdef CONFIG_DEBUG_VM +static void spb_debug_check(struct superpageblock *sb, const char *caller) +{ + u16 total = sb->total_pageblocks; + + VM_WARN_ONCE(sb->nr_free > total, + "%s: nr_free %u > total %u (zone=%s sb=%lu)\n", + caller, sb->nr_free, total, sb->zone->name, + (unsigned long)(sb - sb->zone->superpageblocks)); + VM_WARN_ONCE(sb->nr_unmovable > total, + "%s: nr_unmovable %u > total %u (zone=%s sb=%lu)\n", + caller, sb->nr_unmovable, total, sb->zone->name, + (unsigned long)(sb - sb->zone->superpageblocks)); + VM_WARN_ONCE(sb->nr_reclaimable > total, + "%s: nr_reclaimable %u > total %u (zone=%s sb=%lu)\n", + caller, sb->nr_reclaimable, total, sb->zone->name, + (unsigned long)(sb - sb->zone->superpageblocks)); + VM_WARN_ONCE(sb->nr_movable > total, + "%s: nr_movable %u > total %u (zone=%s sb=%lu)\n", + caller, sb->nr_movable, total, sb->zone->name, + (unsigned long)(sb - sb->zone->superpageblocks)); +} +#else +static inline void spb_debug_check(struct superpageblock *sb, const char *caller) {} +#endif + /* * __spb_set_has_type - set PB_has_* and increment type counter * @@ -552,6 +578,7 @@ static void __spb_set_has_type(struct page *page, int migratetype) sb->nr_movable++; break; } + spb_debug_check(sb, "__spb_set_has_type"); } } @@ -589,6 +616,7 @@ static void __spb_clear_has_type(struct page *page, int migratetype) sb->nr_movable--; break; } + spb_debug_check(sb, "__spb_clear_has_type"); } } @@ -818,6 +846,7 @@ static void superpageblock_pb_now_free(struct page *page) return; sb->nr_free++; + spb_debug_check(sb, "pb_now_free"); spb_update_list(sb); } @@ -840,6 +869,7 @@ static void superpageblock_pb_now_used(struct page *page) if (sb->nr_free) sb->nr_free--; + spb_debug_check(sb, "pb_now_used"); spb_update_list(sb); } @@ -1305,7 +1335,9 @@ static inline void __add_to_free_list(struct page *page, struct zone *zone, struct free_area *area = pfn_sb_free_area(zone, pfn, order, &sb); int nr_pages = 1 << order; - VM_WARN_ONCE(get_pageblock_migratetype(page) != migratetype, + VM_WARN_ONCE(get_pageblock_migratetype(page) != migratetype && + !is_migrate_isolate(get_pageblock_migratetype(page)) && + !is_migrate_cma(get_pageblock_migratetype(page)), "page type is %d, passed migratetype is %d (nr=%d)\n", get_pageblock_migratetype(page), migratetype, nr_pages); @@ -1339,7 +1371,8 @@ static inline void move_to_free_list(struct page *page, struct zone *zone, int nr_pages = 1 << order; /* Free page moving can fail, so it happens before the type update */ - VM_WARN_ONCE(get_pageblock_migratetype(page) != old_mt, + VM_WARN_ONCE(get_pageblock_migratetype(page) != old_mt && + !is_migrate_cma(get_pageblock_migratetype(page)), "page type is %d, passed migratetype is %d (nr=%d)\n", get_pageblock_migratetype(page), old_mt, nr_pages); @@ -1364,7 +1397,9 @@ static inline void __del_page_from_free_list(struct page *page, struct zone *zon struct free_area *area = pfn_sb_free_area(zone, pfn, order, &sb); int nr_pages = 1 << order; - VM_WARN_ONCE(get_pageblock_migratetype(page) != migratetype, + VM_WARN_ONCE(get_pageblock_migratetype(page) != migratetype && + !is_migrate_isolate(get_pageblock_migratetype(page)) && + !is_migrate_cma(get_pageblock_migratetype(page)), "page type is %d, passed migratetype is %d (nr=%d)\n", get_pageblock_migratetype(page), migratetype, nr_pages); @@ -10529,11 +10564,12 @@ static int superpageblock_debugfs_show(struct seq_file *m, void *v) i, sb->start_pfn, sb->total_pageblocks); else - seq_printf(m, " sb[%lu] pfn=0x%lx: unmov=%u recl=%u mov=%u rsv=%u free=%u total=%u\n", + seq_printf(m, " sb[%lu] pfn=0x%lx: unmov=%u recl=%u mov=%u rsv=%u free=%u total=%u free_pages=%lu\n", i, sb->start_pfn, sb->nr_unmovable, sb->nr_reclaimable, sb->nr_movable, sb->nr_reserved, - sb->nr_free, sb->total_pageblocks); + sb->nr_free, sb->total_pageblocks, + sb->nr_free_pages); } } return 0; -- 2.52.0