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

[David Hildenbrand <david@xxxxxxxxxx>]

__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;
        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