Re: [PATCH 1/3] mm: page_alloc: don't steal single pages from biggest buddy

Brendan Jackman
2025-02-25T13:34:41Z

On Mon, Feb 23, 2025 at 07:08:24PM -0500, Johannes Weiner wrote:
> 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.

Nit - I think the try_to_steal_block() changes could be a separate
patch, the history might be easier to understand if it went:

[1/N] mm: page_alloc: don't steal single pages from biggest buddy
[2/N] mm: page_alloc: drop unused logic in steal_suitable_fallback()

(But not a big deal, it's not that hard to follow as-is).

> static __always_inline struct page *
> __rmqueue_fallback(struct zone *zone, int order, int start_migratetype,
> @@ -2291,45 +2289,35 @@ __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;

Is this a separate change? Is it a bug that we currently allow
stealing a from a fallback type when ALLOC_NOFRAGMENT? (I wonder if
the second loop was supposed to start from min_order).

>
> -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);