The fallback code searches for the biggest buddy first in an attempt to steal the whole block and encourage type grouping down the line.
The approach used to be this:
- Non-movable requests will split the largest buddy and steal the remainder. This splits up contiguity, but it allows subsequent requests of this type to fall back into adjacent space.
- Movable requests go and look for the smallest buddy instead. The thinking is that movable requests can be compacted, so grouping is less important than retaining contiguity.
c0cd6f557b90 ("mm: page_alloc: fix freelist movement during block conversion") enforces freelist type hygiene, which restricts stealing to either claiming the whole block or just taking the requested chunk; no additional pages or buddy remainders can be stolen any more.
The patch mishandled when to switch to finding the smallest buddy in that new reality. As a result, it may steal the exact request size, but from the biggest buddy. This causes fracturing for no good reason.
Fix this by committing to the new behavior: either steal the whole block, or fall back to the smallest buddy.
Remove single-page stealing from steal_suitable_fallback(). Rename it to try_to_steal_block() to make the intentions clear. If this fails, always fall back to the smallest buddy.
The following is from 4 runs of mmtest's thpchallenge. "Pollute" is single page fallback, "steal" is conversion of a partially used block. The numbers for free block conversions (omitted) are comparable.
vanilla patched
@pollute[unmovable from reclaimable]: 27 106 @pollute[unmovable from movable]: 82 46 @pollute[reclaimable from unmovable]: 256 83 @pollute[reclaimable from movable]: 46 8 @pollute[movable from unmovable]: 4841 868 @pollute[movable from reclaimable]: 5278 12568
@steal[unmovable from reclaimable]: 11 12 @steal[unmovable from movable]: 113 49 @steal[reclaimable from unmovable]: 19 34 @steal[reclaimable from movable]: 47 21 @steal[movable from unmovable]: 250 183 @steal[movable from reclaimable]: 81 93
The allocator appears to do a better job at keeping stealing and polluting to the first fallback preference. As a result, the numbers for "from movable" - the least preferred fallback option, and most detrimental to compactability - are down across the board.
Link: https://lkml.kernel.org/r/20250225001023.1494422-2-hannes@cmpxchg.org Fixes: c0cd6f557b90 ("mm: page_alloc: fix freelist movement during block conversion") Signed-off-by: Johannes Weiner hannes@cmpxchg.org Suggested-by: Vlastimil Babka vbabka@suse.cz Reviewed-by: Brendan Jackman jackmanb@google.com Reviewed-by: Vlastimil Babka vbabka@suse.cz Signed-off-by: Andrew Morton akpm@linux-foundation.org (cherry picked from commit c2f6ea38fc1b640aa7a2e155cc1c0410ff91afa2) Signed-off-by: Johannes Weiner hannes@cmpxchg.org --- mm/page_alloc.c | 80 +++++++++++++++++++++---------------------------- 1 file changed, 34 insertions(+), 46 deletions(-)
diff --git a/mm/page_alloc.c b/mm/page_alloc.c index 542d25f77be8..6f6c18336cad 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -1908,13 +1908,12 @@ static inline bool boost_watermark(struct zone *zone) * can claim the whole pageblock for the requested migratetype. If not, we check * the pageblock for constituent pages; if at least half of the pages are free * or compatible, we can still claim the whole block, so pages freed in the - * future will be put on the correct free list. Otherwise, we isolate exactly - * the order we need from the fallback block and leave its migratetype alone. + * future will be put on the correct free list. */ static struct page * -steal_suitable_fallback(struct zone *zone, struct page *page, - int current_order, int order, int start_type, - unsigned int alloc_flags, bool whole_block) +try_to_steal_block(struct zone *zone, struct page *page, + int current_order, int order, int start_type, + unsigned int alloc_flags) { int free_pages, movable_pages, alike_pages; unsigned long start_pfn; @@ -1927,7 +1926,7 @@ steal_suitable_fallback(struct zone *zone, struct page *page, * highatomic accounting. */ if (is_migrate_highatomic(block_type)) - goto single_page; + return NULL;
/* Take ownership for orders >= pageblock_order */ if (current_order >= pageblock_order) { @@ -1948,14 +1947,10 @@ steal_suitable_fallback(struct zone *zone, struct page *page, if (boost_watermark(zone) && (alloc_flags & ALLOC_KSWAPD)) set_bit(ZONE_BOOSTED_WATERMARK, &zone->flags);
- /* We are not allowed to try stealing from the whole block */ - if (!whole_block) - goto single_page; - /* moving whole block can fail due to zone boundary conditions */ if (!prep_move_freepages_block(zone, page, &start_pfn, &free_pages, &movable_pages)) - goto single_page; + return NULL;
/* * Determine how many pages are compatible with our allocation. @@ -1988,9 +1983,7 @@ steal_suitable_fallback(struct zone *zone, struct page *page, return __rmqueue_smallest(zone, order, start_type); }
-single_page: - page_del_and_expand(zone, page, order, current_order, block_type); - return page; + return NULL; }
/* @@ -2172,14 +2165,19 @@ static bool unreserve_highatomic_pageblock(const struct alloc_context *ac, }
/* - * Try finding a free buddy page on the fallback list and put it on the free - * list of requested migratetype, possibly along with other pages from the same - * block, depending on fragmentation avoidance heuristics. Returns true if - * fallback was found so that __rmqueue_smallest() can grab it. + * Try finding a free buddy page on the fallback list. + * + * This will attempt to steal a whole pageblock for the requested type + * to ensure grouping of such requests in the future. + * + * If a whole block cannot be stolen, regress to __rmqueue_smallest() + * logic to at least break up as little contiguity as possible. * * The use of signed ints for order and current_order is a deliberate * deviation from the rest of this file, to make the for loop * condition simpler. + * + * Return the stolen page, or NULL if none can be found. */ static __always_inline struct page * __rmqueue_fallback(struct zone *zone, int order, int start_migratetype, @@ -2213,45 +2211,35 @@ __rmqueue_fallback(struct zone *zone, int order, int start_migratetype, if (fallback_mt == -1) continue;
- /* - * We cannot steal all free pages from the pageblock and the - * requested migratetype is movable. In that case it's better to - * steal and split the smallest available page instead of the - * largest available page, because even if the next movable - * allocation falls back into a different pageblock than this - * one, it won't cause permanent fragmentation. - */ - if (!can_steal && start_migratetype == MIGRATE_MOVABLE - && current_order > order) - goto find_smallest; + if (!can_steal) + break;
- goto do_steal; + page = get_page_from_free_area(area, fallback_mt); + page = try_to_steal_block(zone, page, current_order, order, + start_migratetype, alloc_flags); + if (page) + goto got_one; }
- return NULL; + if (alloc_flags & ALLOC_NOFRAGMENT) + return NULL;
-find_smallest: + /* No luck stealing blocks. Find the smallest fallback page */ for (current_order = order; current_order < NR_PAGE_ORDERS; current_order++) { area = &(zone->free_area[current_order]); fallback_mt = find_suitable_fallback(area, current_order, start_migratetype, false, &can_steal); - if (fallback_mt != -1) - break; - } - - /* - * This should not happen - we already found a suitable fallback - * when looking for the largest page. - */ - VM_BUG_ON(current_order > MAX_PAGE_ORDER); + if (fallback_mt == -1) + continue;
-do_steal: - page = get_page_from_free_area(area, fallback_mt); + page = get_page_from_free_area(area, fallback_mt); + page_del_and_expand(zone, page, order, current_order, fallback_mt); + goto got_one; + }
- /* take off list, maybe claim block, expand remainder */ - page = steal_suitable_fallback(zone, page, current_order, order, - start_migratetype, alloc_flags, can_steal); + return NULL;
+got_one: trace_mm_page_alloc_extfrag(page, order, current_order, start_migratetype, fallback_mt);