Re: [PATCH RFC 2/4] mm/page_alloc: place pages to tail in __putback_isolated_page()

Wei Yang
2020-09-18T02:53:03Z

On Wed, Sep 16, 2020 at 08:34:09PM +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.
>
>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

The code looks good, while I don't fully understand the log here.

undo_isolate_page_range() is used in __offline_pages and alloc_contig_range. I
don't connect them with online_pages(). Do I miss something?

>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 <akpm@linux-foundation.org>
>Cc: Alexander Duyck <alexander.h.duyck@linux.intel.com>
>Cc: Mel Gorman <mgorman@techsingularity.net>
>Cc: Michal Hocko <mhocko@kernel.org>
>Cc: Dave Hansen <dave.hansen@intel.com>
>Cc: Vlastimil Babka <vbabka@suse.cz>
>Cc: Wei Yang <richard.weiyang@linux.alibaba.com>
>Cc: Oscar Salvador <osalvador@suse.de>
>Cc: Mike Rapoport <rppt@kernel.org>
>Cc: Scott Cheloha <cheloha@linux.ibm.com>
>Cc: Michael Ellerman <mpe@ellerman.id.au>
>Signed-off-by: David Hildenbrand <david@redhat.com>
>---
> 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

--
Wei Yang
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