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

[Wei Yang]

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
>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@xxxxxxxxxxxxxxxxxxxx>
>Cc: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>
>Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
>Cc: Michal Hocko <mhocko@xxxxxxxxxx>
>Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
>Cc: Vlastimil Babka <vbabka@xxxxxxx>
>Cc: Wei Yang <richard.weiyang@xxxxxxxxxxxxxxxxx>
>Cc: Oscar Salvador <osalvador@xxxxxxx>
>Cc: Mike Rapoport <rppt@xxxxxxxxxx>
>Cc: Scott Cheloha <cheloha@xxxxxxxxxxxxx>
>Cc: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
>Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
>---
> 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;

Take another look into this part. Maybe we can move this check at top?

For online_page case, currently we have following call flow:

    online_page
        online_pages_range
	shuffle_zone

This means we would always shuffle the newly added pages. Maybe we don't need
to do the shuffle when adding them to the free_list?

> 	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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