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Subject[patch 3/5] mm: try to distribute dirty pages fairly across zones
The maximum number of dirty pages that exist in the system at any time
is determined by a number of pages considered dirtyable and a
user-configured percentage of those, or an absolute number in bytes.

This number of dirtyable pages is the sum of memory provided by all
the zones in the system minus their lowmem reserves and high
watermarks, so that the system can retain a healthy number of free
pages without having to reclaim dirty pages.

But there is a flaw in that we have a zoned page allocator which does
not care about the global state but rather the state of individual
memory zones. And right now there is nothing that prevents one zone
from filling up with dirty pages while other zones are spared, which
frequently leads to situations where kswapd, in order to restore the
watermark of free pages, does indeed have to write pages from that
zone's LRU list. This can interfere so badly with IO from the flusher
threads that major filesystems (btrfs, xfs, ext4) mostly ignore write
requests from reclaim already, taking away the VM's only possibility
to keep such a zone balanced, aside from hoping the flushers will soon
clean pages from that zone.

Enter per-zone dirty limits. They are to a zone's dirtyable memory
what the global limit is to the global amount of dirtyable memory, and
try to make sure that no single zone receives more than its fair share
of the globally allowed dirty pages in the first place. As the number
of pages considered dirtyable exclude the zones' lowmem reserves and
high watermarks, the maximum number of dirty pages in a zone is such
that the zone can always be balanced without requiring page cleaning.

As this is a placement decision in the page allocator and pages are
dirtied only after the allocation, this patch allows allocators to
pass __GFP_WRITE when they know in advance that the page will be
written to and become dirty soon. The page allocator will then
attempt to allocate from the first zone of the zonelist - which on
NUMA is determined by the task's NUMA memory policy - that has not
exceeded its dirty limit.

At first glance, it would appear that the diversion to lower zones can
increase pressure on them, but this is not the case. With a full high
zone, allocations will be diverted to lower zones eventually, so it is
more of a shift in timing of the lower zone allocations. Workloads
that previously could fit their dirty pages completely in the higher
zone may be forced to allocate from lower zones, but the amount of
pages that 'spill over' are limited themselves by the lower zones'
dirty constraints, and thus unlikely to become a problem.

For now, the problem of unfair dirty page distribution remains for
NUMA configurations where the zones allowed for allocation are in sum
not big enough to trigger the global dirty limits, wake up the flusher
threads and remedy the situation. Because of this, an allocation that
could not succeed on any of the considered zones is allowed to ignore
the dirty limits before going into direct reclaim or even failing the
allocation, until a future patch changes the global dirty throttling
and flusher thread activation so that they take individual zone states
into account.

Signed-off-by: Johannes Weiner <>
Reviewed-by: Minchan Kim <>
Acked-by: Mel Gorman <>
include/linux/gfp.h | 4 ++-
include/linux/writeback.h | 1 +
mm/page-writeback.c | 83 +++++++++++++++++++++++++++++++++++++++++++++
mm/page_alloc.c | 29 ++++++++++++++++
4 files changed, 116 insertions(+), 1 deletions(-)

diff --git a/include/linux/gfp.h b/include/linux/gfp.h
index 3a76faf..50efc7e 100644
--- a/include/linux/gfp.h
+++ b/include/linux/gfp.h
@@ -36,6 +36,7 @@ struct vm_area_struct;
#define ___GFP_NO_KSWAPD 0x400000u
#define ___GFP_OTHER_NODE 0x800000u
+#define ___GFP_WRITE 0x1000000u

* GFP bitmasks..
@@ -85,6 +86,7 @@ struct vm_area_struct;

#define __GFP_NO_KSWAPD ((__force gfp_t)___GFP_NO_KSWAPD)
#define __GFP_OTHER_NODE ((__force gfp_t)___GFP_OTHER_NODE) /* On behalf of other node */
+#define __GFP_WRITE ((__force gfp_t)___GFP_WRITE) /* Allocator intends to dirty page */

* This may seem redundant, but it's a way of annotating false positives vs.
@@ -92,7 +94,7 @@ struct vm_area_struct;

-#define __GFP_BITS_SHIFT 24 /* Room for N __GFP_FOO bits */
+#define __GFP_BITS_SHIFT 25 /* Room for N __GFP_FOO bits */
#define __GFP_BITS_MASK ((__force gfp_t)((1 << __GFP_BITS_SHIFT) - 1))

/* This equals 0, but use constants in case they ever change */
diff --git a/include/linux/writeback.h b/include/linux/writeback.h
index a5f495f..c96ee0c 100644
--- a/include/linux/writeback.h
+++ b/include/linux/writeback.h
@@ -104,6 +104,7 @@ void laptop_mode_timer_fn(unsigned long data);
static inline void laptop_sync_completion(void) { }
void throttle_vm_writeout(gfp_t gfp_mask);
+bool zone_dirty_ok(struct zone *zone);

extern unsigned long global_dirty_limit;

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 78604a6..f60fd57 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -159,6 +159,25 @@ static struct prop_descriptor vm_dirties;
* We make sure that the background writeout level is below the adjusted
* clamping level.
+ * In a memory zone, there is a certain amount of pages we consider
+ * available for the page cache, which is essentially the number of
+ * free and reclaimable pages, minus some zone reserves to protect
+ * lowmem and the ability to uphold the zone's watermarks without
+ * requiring writeback.
+ *
+ * This number of dirtyable pages is the base value of which the
+ * user-configurable dirty ratio is the effictive number of pages that
+ * are allowed to be actually dirtied. Per individual zone, or
+ * globally by using the sum of dirtyable pages over all zones.
+ *
+ * Because the user is allowed to specify the dirty limit globally as
+ * absolute number of bytes, calculating the per-zone dirty limit can
+ * require translating the configured limit into a percentage of
+ * global dirtyable memory first.
+ */
static unsigned long highmem_dirtyable_memory(unsigned long total)
@@ -245,6 +264,70 @@ void global_dirty_limits(unsigned long *pbackground, unsigned long *pdirty)
trace_global_dirty_state(background, dirty);

+ * zone_dirtyable_memory - number of dirtyable pages in a zone
+ * @zone: the zone
+ *
+ * Returns the zone's number of pages potentially available for dirty
+ * page cache. This is the base value for the per-zone dirty limits.
+ */
+static unsigned long zone_dirtyable_memory(struct zone *zone)
+ /*
+ * The effective global number of dirtyable pages may exclude
+ * highmem as a big-picture measure to keep the ratio between
+ * dirty memory and lowmem reasonable.
+ *
+ * But this function is purely about the individual zone and a
+ * highmem zone can hold its share of dirty pages, so we don't
+ * care about vm_highmem_is_dirtyable here.
+ */
+ return zone_page_state(zone, NR_FREE_PAGES) +
+ zone_reclaimable_pages(zone) -
+ zone->dirty_balance_reserve;
+ * zone_dirty_limit - maximum number of dirty pages allowed in a zone
+ * @zone: the zone
+ *
+ * Returns the maximum number of dirty pages allowed in a zone, based
+ * on the zone's dirtyable memory.
+ */
+static unsigned long zone_dirty_limit(struct zone *zone)
+ unsigned long zone_memory = zone_dirtyable_memory(zone);
+ struct task_struct *tsk = current;
+ unsigned long dirty;
+ if (vm_dirty_bytes)
+ dirty = DIV_ROUND_UP(vm_dirty_bytes, PAGE_SIZE) *
+ zone_memory / global_dirtyable_memory();
+ else
+ dirty = vm_dirty_ratio * zone_memory / 100;
+ if (tsk->flags & PF_LESS_THROTTLE || rt_task(tsk))
+ dirty += dirty / 4;
+ return dirty;
+ * zone_dirty_ok - tells whether a zone is within its dirty limits
+ * @zone: the zone to check
+ *
+ * Returns %true when the dirty pages in @zone are within the zone's
+ * dirty limit, %false if the limit is exceeded.
+ */
+bool zone_dirty_ok(struct zone *zone)
+ unsigned long limit = zone_dirty_limit(zone);
+ return zone_page_state(zone, NR_FILE_DIRTY) +
+ zone_page_state(zone, NR_UNSTABLE_NFS) +
+ zone_page_state(zone, NR_WRITEBACK) <= limit;
* couple the period to the dirty_ratio:
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index f8cba89..afaf59e 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1675,6 +1675,35 @@ zonelist_scan:
if ((alloc_flags & ALLOC_CPUSET) &&
!cpuset_zone_allowed_softwall(zone, gfp_mask))
+ /*
+ * When allocating a page cache page for writing, we
+ * want to get it from a zone that is within its dirty
+ * limit, such that no single zone holds more than its
+ * proportional share of globally allowed dirty pages.
+ * The dirty limits take into account the zone's
+ * lowmem reserves and high watermark so that kswapd
+ * should be able to balance it without having to
+ * write pages from its LRU list.
+ *
+ * This may look like it could increase pressure on
+ * lower zones by failing allocations in higher zones
+ * before they are full. But the pages that do spill
+ * over are limited as the lower zones are protected
+ * by this very same mechanism. It should not become
+ * a practical burden to them.
+ *
+ * XXX: For now, allow allocations to potentially
+ * exceed the per-zone dirty limit in the slowpath
+ * (ALLOC_WMARK_LOW unset) before going into reclaim,
+ * which is important when on a NUMA setup the allowed
+ * zones are together not big enough to reach the
+ * global limit. The proper fix for these situations
+ * will require awareness of zones in the
+ * dirty-throttling and the flusher threads.
+ */
+ if ((alloc_flags & ALLOC_WMARK_LOW) &&
+ (gfp_mask & __GFP_WRITE) && !zone_dirty_ok(zone))
+ goto this_zone_full;

if (!(alloc_flags & ALLOC_NO_WATERMARKS)) {

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