__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. 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). Note: If we observe a degradation due to the changed page isolation behavior (which I doubt), we can always make this configurable by the instance triggering undo of isolation (e.g., alloc_contig_range(), memory onlining, memory offlining). Cc: Andrew Morton <[email protected]> Cc: Alexander Duyck <[email protected]> Cc: Mel Gorman <[email protected]> Cc: Michal Hocko <[email protected]> Cc: Dave Hansen <[email protected]> Cc: Vlastimil Babka <[email protected]> Cc: Wei Yang <[email protected]> Cc: Oscar Salvador <[email protected]> Cc: Mike Rapoport <[email protected]> Cc: Scott Cheloha <[email protected]> Cc: Michael Ellerman <[email protected]> Signed-off-by: David Hildenbrand <[email protected]> --- mm/page_alloc.c | 10 +++++++++- 1 file changed, 9 insertions(+), 1 deletion(-) diff --git a/mm/page_alloc.c b/mm/page_alloc.c index 91cefb8157dd..bba9a0f60c70 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -89,6 +89,12 @@ typedef int __bitwise fop_t; */ #define FOP_SKIP_REPORT_NOTIFY ((__force fop_t)BIT(0)) +/* + * Place the freed page to the tail of the freelist after buddy merging. Will + * get ignored with page shuffling enabled. + */ +#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); #define MIN_PERCPU_PAGELIST_FRACTION (8) @@ -1040,6 +1046,8 @@ static inline void __free_one_page(struct page *page, unsigned long pfn, if (is_shuffle_order(order)) to_tail = shuffle_pick_tail(); + else if (fop_flags & FOP_TO_TAIL) + to_tail = true; else to_tail = buddy_merge_likely(pfn, buddy_pfn, page, order); @@ -3289,7 +3297,7 @@ 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, - FOP_SKIP_REPORT_NOTIFY); + FOP_SKIP_REPORT_NOTIFY | FOP_TO_TAIL); } /* -- 2.26.2
