[RFC PATCH 26/45] mm: page_alloc: prevent UNMOVABLE/RECLAIMABLE mixing in pageblocks

Rik van Riel
2026-04-30T20:22:50Z

Summary:
Inside a tainted SPB, free pages of UNMOVABLE and RECLAIMABLE
allocations cannot be told apart by the buddy allocator's
compatibility heuristic (alike_pages == 0 between the two non-movable
types in try_to_claim_block). Once a pageblock holds in-use pages of
both, any sticky UNMOVABLE pinhole prevents the RECLAIMABLE pages
from coalescing into useful higher-order chunks when they drain back
to the buddy. The PB's free capacity is permanently capped at
order-1 dust regardless of how much of it actually returns. Sticky
recl pages (active dentries, locked btrfs eb folios, NOFS slab) are
unavoidable; the cost is paid in internal fragmentation.

Two paths in the page allocator create UNMOVABLE<->RECLAIMABLE
mixing today:

1. try_to_claim_block() relabels a partial PB whenever the 50%
threshold "free_pages + alike_pages >= pageblock_nr_pages/2"
passes. For UNMOV<->RECL, alike_pages == 0, so the rule
degenerates to free_pages >= 256. A PB with 256 in-use UNMOV
pages plus 256 free pages passes and is relabeled RECL. Both
PB_has_unmovable and PB_has_reclaimable are then set.

2. __rmqueue_steal() takes a single foreign-type page out of a
PB without relabeling the PB. A UNMOVABLE allocation stealing
from a RECLAIMABLE-labeled PB sets PB_has_unmovable on top of
the existing PB_has_reclaimable.

Tighten both paths:

- Add noncompatible_cross_type() helper that detects the
UNMOV<->RECL pair (MOVABLE may still mix with either since
movable pages can be migrated out).

- In try_to_claim_block(), require a fully-free PB
(free_pages == pageblock_nr_pages) for any cross-type relabel,
regardless of from_tainted_spb. The other-type bit inherited
from the prior label is stale on a fully-free PB (no in-use
pages of either type) so clear it during the relabel rather
than leaving the PB visibly mixed in PB_has_* state.

- In __rmqueue_steal(), pass a new SB_SKIP_CROSS_TYPE flag to
__rmqueue_sb_find_fallback() so the cross-type fallback entry
in fallbacks[] is skipped. Steal then falls through to the
MIGRATE_MOVABLE second fallback instead of single-page-stealing
into a foreign non-movable PB.

The from_tainted_spb=true caller of try_to_claim_block() is
unaffected because it hardcodes block_type=MIGRATE_MOVABLE. The
claim_whole_block() branch (current_order >= pageblock_order) is
also unaffected: it requires PB_all_free, so the PB is fully free
of any prior type.

Test Plan:
Bare-metal devvm with the existing 4 stuck tainted SPBs (sb[2,15,
36,51] in Normal). Build and reboot. Compare per-order free
distribution in newly tainted SPBs against pre-patch baseline:
today o0/o1 dominate, target meaningful (>10%) free at order >= 3
in pure-RECL SPBs created post-patch. Watch for tainted SPB count
growth past ~12 (3x current baseline) — the fully-free constraint
on cross-type claims will taint fresh SPBs more often, and a
runaway count means the cost was misjudged. Watch dmesg for
allocation failures and kswapd CPU stays under 2 cores. Existing
mixed SPBs from before this change won't unmix; the win is for
SPBs created after.

Reviewers:

Subscribers:

Tasks:

Tags:

Signed-off-by: Rik van Riel <riel@surriel.com>
Assisted-by: Claude:claude-opus-4.7 syzkaller
---
mm/page_alloc.c | 111 ++++++++++++++++++++++++++++++++++++------------
1 file changed, 85 insertions(+), 26 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 67cc8165ab1f..ceb1284a63ed 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3057,6 +3057,23 @@ static int fallbacks[MIGRATE_PCPTYPES][MIGRATE_PCPTYPES - 1] = {
[MIGRATE_RECLAIMABLE] = { MIGRATE_UNMOVABLE, MIGRATE_MOVABLE },
};

+/*
+ * UNMOVABLE and RECLAIMABLE allocations should not share the same
+ * pageblock. Their free pages are interchangeable on the buddy free
+ * lists (alike_pages == 0 between them), so once a PB holds both
+ * types the buddy can no longer tell them apart and any sticky
+ * UNMOVABLE pinhole prevents the RECLAIMABLE pages from coalescing
+ * into useful higher-order chunks when they drain back. MOVABLE may
+ * mix with either, since MOVABLE pages can be migrated out.
+ */
+static inline bool noncompatible_cross_type(int start_type, int fallback_type)
+{
+ return (start_type == MIGRATE_UNMOVABLE &&
+ fallback_type == MIGRATE_RECLAIMABLE) ||
+ (start_type == MIGRATE_RECLAIMABLE &&
+ fallback_type == MIGRATE_UNMOVABLE);
+}
+
#ifdef CONFIG_CMA
static __always_inline struct page *__rmqueue_cma_fallback(struct zone *zone,
unsigned int order)
@@ -3434,6 +3451,9 @@ try_to_claim_block(struct zone *zone, struct page *page,
bool from_tainted_spb)
{
int free_pages, movable_pages, alike_pages;
+#ifdef CONFIG_COMPACTION
+ struct superpageblock *sb;
+#endif
unsigned long start_pfn;

/*
@@ -3492,35 +3512,48 @@ try_to_claim_block(struct zone *zone, struct page *page,
* allocations. Inside a tainted SPB the protection is unnecessary:
* fragmentation has already been accepted at the SPB level, and
* relabeling is much cheaper than tainting a fresh clean SPB.
- */
- if (from_tainted_spb ||
- free_pages + alike_pages >= (1 << (pageblock_order-1)) ||
- page_group_by_mobility_disabled) {
- __move_freepages_block(zone, start_pfn, block_type, start_type);
- set_pageblock_migratetype(pfn_to_page(start_pfn), start_type);
-#ifdef CONFIG_COMPACTION
- /*
- * Track actual page contents in pageblock flags and
- * update superpageblock counters so the SPB moves to
- * the correct fullness list for steering.
- */
- {
- struct page *start_page = pfn_to_page(start_pfn);
- struct superpageblock *sb;
-
- __spb_set_has_type(start_page, start_type);
- if (block_type != start_type)
- __spb_set_has_type(start_page, block_type);
+ *
+ * UNMOVABLE<->RECLAIMABLE cross-type claims override these rules:
+ * once mixed, sticky pinholes of one type prevent the other from
+ * coalescing into useful higher-order free chunks even after drain.
+ * Only relabel a fully-free PB in that case, regardless of whether
+ * the SPB is tainted.
+ */
+ if (noncompatible_cross_type(start_type, block_type)) {
+ if (free_pages != pageblock_nr_pages)
+ return NULL;
+ } else if (!from_tainted_spb &&
+ free_pages + alike_pages < (1 << (pageblock_order-1)) &&
+ !page_group_by_mobility_disabled) {
+ return NULL;
+ }

- sb = pfn_to_superpageblock(zone, start_pfn);
- if (sb)
- spb_update_list(sb);
- }
-#endif
- return __rmqueue_smallest(zone, order, start_type);
+ __move_freepages_block(zone, start_pfn, block_type, start_type);
+ set_pageblock_migratetype(pfn_to_page(start_pfn), start_type);
+#ifdef CONFIG_COMPACTION
+ /*
+ * Track actual page contents in pageblock flags and update
+ * superpageblock counters so the SPB moves to the correct
+ * fullness list for steering.
+ *
+ * For cross-type UNMOVABLE<->RECLAIMABLE relabel (which by the
+ * predicate above only fires on a fully-free PB), the inherited
+ * PB_has_<block_type> bit is stale — there are no in-use pages
+ * of that type. Clear it so the resulting PB is unmixed.
+ */
+ __spb_set_has_type(pfn_to_page(start_pfn), start_type);
+ if (block_type != start_type) {
+ if (noncompatible_cross_type(start_type, block_type))
+ __spb_clear_has_type(pfn_to_page(start_pfn), block_type);
+ else
+ __spb_set_has_type(pfn_to_page(start_pfn), block_type);
}

- return NULL;
+ sb = pfn_to_superpageblock(zone, start_pfn);
+ if (sb)
+ spb_update_list(sb);
+#endif
+ return __rmqueue_smallest(zone, order, start_type);
}

/*
@@ -3544,6 +3577,13 @@ try_to_claim_block(struct zone *zone, struct page *page,
#define SB_SEARCH_EMPTY (1 << 1)
#define SB_SEARCH_FALLBACK (1 << 2)
#define SB_SEARCH_ALL (SB_SEARCH_PREFERRED | SB_SEARCH_EMPTY | SB_SEARCH_FALLBACK)
+/*
+ * Skip UNMOVABLE<->RECLAIMABLE cross-type fallback. Used by the steal
+ * path to prevent landing single foreign-type pages into a PB labeled
+ * with the other non-movable type — a steal does not relabel the PB
+ * so cross-type stealing creates permanent mixing.
+ */
+#define SB_SKIP_CROSS_TYPE (1 << 3)

static struct page *
__rmqueue_sb_find_fallback(struct zone *zone, unsigned int order,
@@ -3580,6 +3620,10 @@ __rmqueue_sb_find_fallback(struct zone *zone, unsigned int order,
int fmt = fallbacks[start_migratetype][i];
struct page *page;

+ if ((search_cats & SB_SKIP_CROSS_TYPE) &&
+ noncompatible_cross_type(start_migratetype, fmt))
+ continue;
+
page = get_page_from_free_area(area,
fmt);
if (page) {
@@ -3601,6 +3645,10 @@ __rmqueue_sb_find_fallback(struct zone *zone, unsigned int order,
int fmt = fallbacks[start_migratetype][i];
struct page *page;

+ if ((search_cats & SB_SKIP_CROSS_TYPE) &&
+ noncompatible_cross_type(start_migratetype, fmt))
+ continue;
+
page = get_page_from_free_area(area,
fmt);
if (page) {
@@ -3629,6 +3677,10 @@ __rmqueue_sb_find_fallback(struct zone *zone, unsigned int order,
int fmt = fallbacks[start_migratetype][i];
struct page *page;

+ if ((search_cats & SB_SKIP_CROSS_TYPE) &&
+ noncompatible_cross_type(start_migratetype, fmt))
+ continue;
+
page = get_page_from_free_area(area,
fmt);
if (page) {
@@ -3765,11 +3817,18 @@ __rmqueue_steal(struct zone *zone, int order, int start_migratetype,
/*
* When ALLOC_NOFRAG_TAINTED_OK is set, only steal from tainted
* SPBs to avoid tainting clean ones. Otherwise search all categories.
+ *
+ * Always skip UNMOVABLE<->RECLAIMABLE cross-type fallback. The steal
+ * path takes a single page without relabeling its PB, so a cross-type
+ * steal would land an UNMOVABLE page in a RECLAIMABLE-labeled PB
+ * (or vice versa) and create permanent mixing. Falling through to
+ * MIGRATE_MOVABLE (the second fallback) is preferable.
*/
if (alloc_flags & ALLOC_NOFRAG_TAINTED_OK)
search_cats = SB_SEARCH_PREFERRED;
else
search_cats = SB_SEARCH_PREFERRED | SB_SEARCH_FALLBACK;
+ search_cats |= SB_SKIP_CROSS_TYPE;

/*
* Search per-superpageblock free lists for fallback migratetypes.
--
2.52.0