Add Phase 2 to rmqueue_bulk: when refilling PCP for unmovable or reclaimable allocations, search tainted superpageblocks for partially-free pageblocks with sub-pageblock buddy entries of the requested migratetype. Claim ownership of the pageblock and move the found entry to PCP with PCPBuddy marking. Pass 0 (the existing owned-block recovery phase) picks up remaining buddy entries on subsequent refills, so there is no need to sweep the entire pageblock eagerly. This concentrates non-movable allocations into already-tainted superpageblocks, reducing fragmentation spread to clean superpageblocks. Pageblock-ownership handling: a pageblock encoded as pbd->cpu==0 is unowned and may be claimed; a non-zero value means another CPU's PCP has frozen pages from this block. In the latter case the refill walk keeps following the pageblock (the merge pass at __free_one_page can reabsorb the other CPU's PCPBuddy entries in the same lock acquire, clearing ownership before the walk finishes), instead of unconditionally skipping it. Without this, busy multi-CPU systems with high tainted-SPB occupancy would skip every already-touched pageblock in Phase 2 and let clean SPBs taint instead -- the exact failure Phase 2 was added to prevent. Signed-off-by: Rik van Riel Assisted-by: Claude:claude-opus-4.7 syzkaller --- mm/page_alloc.c | 131 ++++++++++++++++++++++++++++++++++++++++++------ 1 file changed, 117 insertions(+), 14 deletions(-) diff --git a/mm/page_alloc.c b/mm/page_alloc.c index 093be0d930c0..8027412da866 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -1090,7 +1090,7 @@ static inline void set_buddy_order(struct page *page, unsigned int order) * - Set when Phase 0/1 restore or acquire whole pageblocks. * - Propagated to split remainders in pcp_rmqueue_smallest(). * - Set on freed pages from owned blocks routed to the owner PCP. - * - NOT set for Phase 2/3 fragments or zone-owned frees. + * - NOT set for Phase 3 fragments or zone-owned frees. * - The merge pass in free_pcppages_bulk() only processes * PagePCPBuddy pages, ensuring it never touches pages on * another CPU's PCP list. @@ -3871,15 +3871,15 @@ __rmqueue(struct zone *zone, unsigned int order, int migratetype, * under a single hold of the lock, for efficiency. Add them to the * freelist of @pcp. * - * When @pcp is non-NULL and @count > 1 (normal pageset), uses a four-phase + * When @pcp is non-NULL and @count > 1 (normal pageset), uses a multi-phase * approach: - * Phase 0: Recover previously owned, partially drained blocks. - * Phase 1: Acquire whole pageblocks, claim ownership, set PagePCPBuddy. - * These pages are eligible for PCP-level buddy merging. - * Phase 2: Grab sub-pageblock fragments of the same migratetype. - * Phase 3: Fall back to __rmqueue() with migratetype fallback. - * Phase 2/3 pages are cached for batching only -- no ownership claim, - * no PagePCPBuddy, no PCP-level merging. + * Phase 0: Recover previously owned, partially drained blocks. + * Phase 1: Acquire whole pageblocks, claim ownership, set PagePCPBuddy. + * These pages are eligible for PCP-level buddy merging. + * Phase 2: Adopt partial pageblocks from tainted SPBs (non-movable only). + * Claims ownership so Pass 0 can recover buddy entries later. + * Phase 3: Fall back to __rmqueue() with migratetype fallback. + * No ownership claim, no PagePCPBuddy, no PCP-level merging. * * When @pcp is NULL or @count <= 1 (boot pageset), acquires individual * pages of the requested order directly. @@ -3897,7 +3897,7 @@ static bool rmqueue_bulk(struct zone *zone, unsigned int order, int cpu = smp_processor_id(); unsigned long refilled = 0; unsigned long flags; - int o; + unsigned int o; if (unlikely(alloc_flags & ALLOC_TRYLOCK)) { if (!spin_trylock_irqsave(&zone->lock, flags)) @@ -4007,11 +4007,114 @@ static bool rmqueue_bulk(struct zone *zone, unsigned int order, goto out; /* - * Phase 2 was removed: it swept zone free lists for sub-pageblock - * fragments, which are always empty when superpageblocks are enabled. - * Phase 3's __rmqueue() -> __rmqueue_smallest() properly searches - * per-superpageblock free lists at all orders. + * Phase 2: Adopt partial pageblocks from tainted SPBs. + * + * Phase 1 only grabs whole free pageblocks. When a tainted SPB + * has partially-used pageblocks with free sub-pageblock buddy + * entries, Phase 1 can't use them. Phase 3 can find them via + * __rmqueue_smallest, but without ownership or PCPBuddy marking, + * so they fragment further on drain. + * + * This phase bridges the gap: find a sub-pageblock free entry + * in a tainted SPB and claim ownership of its pageblock. Pass 0 + * will pick up remaining buddy entries on subsequent refills. + * + * Only for unmovable/reclaimable -- movable should use clean SPBs. */ + if (migratetype != MIGRATE_MOVABLE && + !is_migrate_cma(migratetype)) { + enum sb_fullness full; + + for (full = SB_FULL; full < __NR_SB_FULLNESS; full++) { + struct superpageblock *sb; + + list_for_each_entry(sb, + &zone->spb_lists[SB_TAINTED][full], list) { + struct page *page; + int found_order = -1; + bool claim_pb; + + if (sb->nr_free_pages < pageblock_nr_pages / 4) + continue; + + /* + * Find a sub-pageblock free entry for our + * migratetype, starting from the largest order. + * + * Use a post-decrement loop so the unsigned + * counter cannot underflow when @order is 0; + * the previous signed counter relied on the + * mixed signed/unsigned comparison wrapping + * to a huge value, which UBSAN flagged and + * which let the loop walk free_area[-1]. + */ + for (o = pageblock_order; o-- > order; ) { + struct free_area *area; + + area = &sb->free_area[o]; + page = get_page_from_free_area( + area, migratetype); + if (page) { + found_order = o; + break; + } + } + if (found_order < 0) + continue; + + /* + * Found a free fragment in a tainted SPB. Take + * it from the buddy. + * + * If the source pageblock is unowned, claim it: + * mark our pages PagePCPBuddy and register the + * block on owned_blocks so Pass 0 can recover + * remaining fragments on future refills. + * + * If the source pageblock is already owned by + * some CPU (us or another), take the page as a + * plain non-PCPBuddy fragment -- the same way + * Phase 3 / __rmqueue_smallest would. Setting + * PagePCPBuddy here would let two CPUs hold + * PCPBuddy pages from the same pageblock, and + * the PCP merge pass could then corrupt the + * other CPU's PCP list. + * + * Set PB_has_ either way (bypasses + * page_del_and_expand which normally does the + * PB_has tracking); idempotent if already set. + */ + pbd = pfn_to_pageblock(page, + page_to_pfn(page)); + claim_pb = (pbd->cpu == 0); + + del_page_from_free_list(page, zone, + found_order, + migratetype); + __spb_set_has_type(page, migratetype); + if (claim_pb) { + set_pcpblock_owner(page, cpu); + __SetPagePCPBuddy(page); + } + pcp_enqueue_tail(pcp, page, migratetype, + found_order); + refilled += 1 << found_order; + + /* + * Register for Phase 0 recovery so future + * drains from this pageblock can be swept + * back efficiently. Only meaningful when we + * actually claimed ownership above. + */ + if (claim_pb && list_empty(&pbd->cpu_node)) + list_add(&pbd->cpu_node, + &pcp->owned_blocks); + + if (refilled >= pages_needed) + goto out; + } + } + } /* * Phase 3: Last resort. Use __rmqueue() which does -- 2.54.0