From: David Hildenbrand <david@xxxxxxxxxx> Subject: mm/page_alloc: place pages to tail in __putback_isolated_page() __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 desirable 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 freelist, 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 rely on this behavior for correction (if the order of the freelists 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 free_contig_range(). Link: https://lkml.kernel.org/r/20201005121534.15649-3-david@xxxxxxxxxx Signed-off-by: David Hildenbrand <david@xxxxxxxxxx> Reviewed-by: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx> Reviewed-by: Oscar Salvador <osalvador@xxxxxxx> Reviewed-by: Wei Yang <richard.weiyang@xxxxxxxxxxxxxxxxx> Reviewed-by: Pankaj Gupta <pankaj.gupta.linux@xxxxxxxxx> Acked-by: Michal Hocko <mhocko@xxxxxxxx> Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> Cc: Dave Hansen <dave.hansen@xxxxxxxxx> Cc: Vlastimil Babka <vbabka@xxxxxxx> Cc: Mike Rapoport <rppt@xxxxxxxxxx> Cc: Scott Cheloha <cheloha@xxxxxxxxxxxxx> Cc: Michael Ellerman <mpe@xxxxxxxxxxxxxx> Cc: Haiyang Zhang <haiyangz@xxxxxxxxxxxxx> Cc: "K. Y. Srinivasan" <kys@xxxxxxxxxxxxx> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx> Cc: Michal Hocko <mhocko@xxxxxxxxxx> Cc: Stephen Hemminger <sthemmin@xxxxxxxxxxxxx> Cc: Wei Liu <wei.liu@xxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- mm/page_alloc.c | 18 ++++++++++++++++-- 1 file changed, 16 insertions(+), 2 deletions(-) --- a/mm/page_alloc.c~mm-page_alloc-place-pages-to-tail-in-__putback_isolated_page +++ a/mm/page_alloc.c @@ -94,6 +94,18 @@ typedef int __bitwise fpi_t; */ #define FPI_SKIP_REPORT_NOTIFY ((__force fpi_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 on 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 FPI_TO_TAIL ((__force fpi_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) @@ -1044,7 +1056,9 @@ continue_merging: done_merging: set_page_order(page, order); - if (is_shuffle_order(order)) + if (fpi_flags & FPI_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); @@ -3306,7 +3320,7 @@ void __putback_isolated_page(struct page /* Return isolated page to tail of freelist. */ __free_one_page(page, page_to_pfn(page), zone, order, mt, - FPI_SKIP_REPORT_NOTIFY); + FPI_SKIP_REPORT_NOTIFY | FPI_TO_TAIL); } /* _
