diff mbox series

[080/156] mm/page_alloc: place pages to tail in __putback_isolated_page()

Message ID 20201016024619.V5RUw7-Zg%akpm@linux-foundation.org
State New
Headers show
Series [001/156] device-dax/kmem: fix resource release | expand

Commit Message

Andrew Morton Oct. 16, 2020, 2:46 a.m. UTC
From: David Hildenbrand <david@redhat.com>
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

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@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Reviewed-by: Wei Yang <richard.weiyang@linux.alibaba.com>
Reviewed-by: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Scott Cheloha <cheloha@linux.ibm.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Haiyang Zhang <haiyangz@microsoft.com>
Cc: "K. Y. Srinivasan" <kys@microsoft.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Stephen Hemminger <sthemmin@microsoft.com>
Cc: Wei Liu <wei.liu@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

 mm/page_alloc.c |   18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)
diff mbox series


--- 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);
@@ -1044,7 +1056,9 @@  continue_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();
 		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,