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SubjectRe: [PATCH v1 2/5] mm/page_alloc: place pages to tail in __putback_isolated_page()
On Mon, Sep 28, 2020 at 08:21:07PM +0200, David Hildenbrand wrote:
>__putback_isolated_page() already documents that pages will be placed to
>the tail of the freelist - this is, however, not the case for
>"order >= MAX_ORDER - 2" (see buddy_merge_likely()) - which should be
>the case for all existing users.
>This change affects two users:
>- free page reporting
>- page isolation, when undoing the isolation (including memory onlining).
>This behavior is desireable for pages that haven't really been touched
>lately, so exactly the two users that don't actually read/write page
>content, but rather move untouched pages.
>The new behavior is especially desirable for memory onlining, where we
>allow allocation of newly onlined pages via undo_isolate_page_range()
>in online_pages(). Right now, we always place them to the head of the
>free list, resulting in undesireable behavior: Assume we add
>individual memory chunks via add_memory() and online them right away to
>the NORMAL zone. We create a dependency chain of unmovable allocations
>e.g., via the memmap. The memmap of the next chunk will be placed onto
>previous chunks - if the last block cannot get offlined+removed, all
>dependent ones cannot get offlined+removed. While this can already be
>observed with individual DIMMs, it's more of an issue for virtio-mem
>(and I suspect also ppc DLPAR).
>Document that this should only be used for optimizations, and no code
>should realy on this for correction (if the order of freepage lists
>ever changes).
>We won't care about page shuffling: memory onlining already properly
>shuffles after onlining. free page reporting doesn't care about
>physically contiguous ranges, and there are already cases where page
>isolation will simply move (physically close) free pages to (currently)
>the head of the freelists via move_freepages_block() instead of
>shuffling. If this becomes ever relevant, we should shuffle the whole
>zone when undoing isolation of larger ranges, and after
>Reviewed-by: Alexander Duyck <>
>Reviewed-by: Oscar Salvador <>
>Cc: Andrew Morton <>
>Cc: Alexander Duyck <>
>Cc: Mel Gorman <>
>Cc: Michal Hocko <>
>Cc: Dave Hansen <>
>Cc: Vlastimil Babka <>
>Cc: Wei Yang <>
>Cc: Oscar Salvador <>
>Cc: Mike Rapoport <>
>Cc: Scott Cheloha <>
>Cc: Michael Ellerman <>
>Signed-off-by: David Hildenbrand <>

Reviewed-by: Wei Yang <>

> mm/page_alloc.c | 18 ++++++++++++++++--
> 1 file changed, 16 insertions(+), 2 deletions(-)
>diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>index daab90e960fe..9e3ed4a6f69a 100644
>--- a/mm/page_alloc.c
>+++ b/mm/page_alloc.c
>@@ -89,6 +89,18 @@ typedef int __bitwise fop_t;
> */
> #define FOP_SKIP_REPORT_NOTIFY ((__force fop_t)BIT(0))
>+ * Place the (possibly merged) page to the tail of the freelist. Will ignore
>+ * page shuffling (relevant code - e.g., memory onlining - is expected to
>+ * shuffle the whole zone).
>+ *
>+ * Note: No code should rely onto this flag for correctness - it's purely
>+ * to allow for optimizations when handing back either fresh pages
>+ * (memory onlining) or untouched pages (page isolation, free page
>+ * reporting).
>+ */
>+#define FOP_TO_TAIL ((__force fop_t)BIT(1))
> /* prevent >1 _updater_ of zone percpu pageset ->high and ->batch fields */
> static DEFINE_MUTEX(pcp_batch_high_lock);
>@@ -1038,7 +1050,9 @@ static inline void __free_one_page(struct page *page, unsigned long pfn,
> done_merging:
> set_page_order(page, order);
>- if (is_shuffle_order(order))
>+ if (fop_flags & FOP_TO_TAIL)
>+ to_tail = true;
>+ else if (is_shuffle_order(order))
> to_tail = shuffle_pick_tail();
> else
> to_tail = buddy_merge_likely(pfn, buddy_pfn, page, order);
>@@ -3300,7 +3314,7 @@ void __putback_isolated_page(struct page *page, unsigned int order, int mt)
> /* Return isolated page to tail of freelist. */
> __free_one_page(page, page_to_pfn(page), zone, order, mt,
> }
> /*

Wei Yang
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