[patch 4/5] mm: writeback: throttle __GFP_WRITE on per-zone dirty limits

[Johannes Weiner <jweiner@redhat.com>]

From: Johannes Weiner <hannes@cmpxchg.org>

Allow allocators to pass __GFP_WRITE when they know in advance that
the allocated page will be written to and become dirty soon.

The page allocator will then attempt to distribute those allocations
across zones, such that no single zone will end up full of dirty and
thus more or less unreclaimable pages.

The global dirty limits are put in proportion to the respective zone's
amount of dirtyable memory and the allocation denied when the limit of
that zone is reached.

Before the allocation fails, the allocator slowpath has a stage before
compaction and reclaim, where the flusher threads are kicked and the
allocator ultimately has to wait for writeback if still none of the
zones has become eligible for allocation again in the meantime.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
---
 include/linux/gfp.h       |    4 +-
 include/linux/writeback.h |    3 +
 mm/page-writeback.c       |  132 +++++++++++++++++++++++++++++++++++++++------
 mm/page_alloc.c           |   27 +++++++++
 4 files changed, 149 insertions(+), 17 deletions(-)

diff --git a/include/linux/gfp.h b/include/linux/gfp.h
index 3a76faf..78d5338 100644
--- a/include/linux/gfp.h
+++ b/include/linux/gfp.h
@@ -36,6 +36,7 @@ struct vm_area_struct;
 #endif
 #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)	/* Will be dirtied soon */
 
 /*
  * This may seem redundant, but it's a way of annotating false positives vs.
@@ -92,7 +94,7 @@ struct vm_area_struct;
  */
 #define __GFP_NOTRACK_FALSE_POSITIVE (__GFP_NOTRACK)
 
-#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 8c63f3a..9312e25 100644
--- a/include/linux/writeback.h
+++ b/include/linux/writeback.h
@@ -93,6 +93,9 @@ void laptop_mode_timer_fn(unsigned long data);
 static inline void laptop_sync_completion(void) { }
 #endif
 void throttle_vm_writeout(gfp_t gfp_mask);
+bool zone_dirty_ok(struct zone *zone);
+void try_to_writeback_pages(struct zonelist *zonelist, gfp_t gfp_mask,
+			    nodemask_t *nodemask);
 
 /* These are exported to sysctl. */
 extern int dirty_background_ratio;
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 41dc871..ce673ec 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -154,6 +154,18 @@ static unsigned long determine_dirtyable_memory(void)
 	return x + 1;	/* Ensure that we never return 0 */
 }
 
+static unsigned long zone_dirtyable_memory(struct zone *zone)
+{
+	unsigned long x = 1; /* Ensure that we never return 0 */
+
+	if (is_highmem(zone) && !vm_highmem_is_dirtyable)
+		return x;
+
+	x += zone_page_state(zone, NR_FREE_PAGES);
+	x += zone_reclaimable_pages(zone);
+	return x;
+}
+
 /*
  * Scale the writeback cache size proportional to the relative writeout speeds.
  *
@@ -378,6 +390,24 @@ int bdi_set_max_ratio(struct backing_dev_info *bdi, unsigned max_ratio)
 }
 EXPORT_SYMBOL(bdi_set_max_ratio);
 
+static void sanitize_dirty_limits(unsigned long *pbackground,
+				  unsigned long *pdirty)
+{
+	unsigned long background = *pbackground;
+	unsigned long dirty = *pdirty;
+	struct task_struct *tsk;
+
+	if (background >= dirty)
+		background = dirty / 2;
+	tsk = current;
+	if (tsk->flags & PF_LESS_THROTTLE || rt_task(tsk)) {
+		background += background / 4;
+		dirty += dirty / 4;
+	}
+	*pbackground = background;
+	*pdirty = dirty;
+}
+
 /*
  * global_dirty_limits - background-writeback and dirty-throttling thresholds
  *
@@ -389,33 +419,52 @@ EXPORT_SYMBOL(bdi_set_max_ratio);
  */
 void global_dirty_limits(unsigned long *pbackground, unsigned long *pdirty)
 {
-	unsigned long background;
-	unsigned long dirty;
 	unsigned long uninitialized_var(available_memory);
-	struct task_struct *tsk;
 
 	if (!vm_dirty_bytes || !dirty_background_bytes)
 		available_memory = determine_dirtyable_memory();
 
 	if (vm_dirty_bytes)
-		dirty = DIV_ROUND_UP(vm_dirty_bytes, PAGE_SIZE);
+		*pdirty = DIV_ROUND_UP(vm_dirty_bytes, PAGE_SIZE);
 	else
-		dirty = (vm_dirty_ratio * available_memory) / 100;
+		*pdirty = vm_dirty_ratio * available_memory / 100;
 
 	if (dirty_background_bytes)
-		background = DIV_ROUND_UP(dirty_background_bytes, PAGE_SIZE);
+		*pbackground = DIV_ROUND_UP(dirty_background_bytes, PAGE_SIZE);
 	else
-		background = (dirty_background_ratio * available_memory) / 100;
+		*pbackground = dirty_background_ratio * available_memory / 100;
 
-	if (background >= dirty)
-		background = dirty / 2;
-	tsk = current;
-	if (tsk->flags & PF_LESS_THROTTLE || rt_task(tsk)) {
-		background += background / 4;
-		dirty += dirty / 4;
-	}
-	*pbackground = background;
-	*pdirty = dirty;
+	sanitize_dirty_limits(pbackground, pdirty);
+}
+
+static void zone_dirty_limits(struct zone *zone, unsigned long *pbackground,
+			      unsigned long *pdirty)
+{
+	unsigned long uninitialized_var(global_memory);
+	unsigned long zone_memory;
+
+	zone_memory = zone_dirtyable_memory(zone);
+
+	if (!vm_dirty_bytes || !dirty_background_bytes)
+		global_memory = determine_dirtyable_memory();
+
+	if (vm_dirty_bytes) {
+		unsigned long dirty_pages;
+
+		dirty_pages = DIV_ROUND_UP(vm_dirty_bytes, PAGE_SIZE);
+		*pdirty = zone_memory * dirty_pages / global_memory;
+	} else
+		*pdirty = zone_memory * vm_dirty_ratio / 100;
+
+	if (dirty_background_bytes) {
+		unsigned long dirty_pages;
+
+		dirty_pages = DIV_ROUND_UP(dirty_background_bytes, PAGE_SIZE);
+		*pbackground = zone_memory * dirty_pages / global_memory;
+	} else
+		*pbackground = zone_memory * dirty_background_ratio / 100;
+
+	sanitize_dirty_limits(pbackground, pdirty);
 }
 
 /*
@@ -661,6 +710,57 @@ void throttle_vm_writeout(gfp_t gfp_mask)
         }
 }
 
+bool zone_dirty_ok(struct zone *zone)
+{
+	unsigned long background_thresh, dirty_thresh;
+	unsigned long nr_reclaimable, nr_writeback;
+
+	zone_dirty_limits(zone, &background_thresh, &dirty_thresh);
+
+	nr_reclaimable = zone_page_state(zone, NR_FILE_DIRTY) +
+		zone_page_state(zone, NR_UNSTABLE_NFS);
+	nr_writeback = zone_page_state(zone, NR_WRITEBACK);
+
+	return nr_reclaimable + nr_writeback <= dirty_thresh;
+}
+
+void try_to_writeback_pages(struct zonelist *zonelist, gfp_t gfp_mask,
+			    nodemask_t *nodemask)
+{
+	unsigned int nr_exceeded = 0;
+	unsigned int nr_zones = 0;
+	struct zoneref *z;
+	struct zone *zone;
+
+	for_each_zone_zonelist_nodemask(zone, z, zonelist, gfp_zone(gfp_mask),
+					nodemask) {
+		unsigned long background_thresh, dirty_thresh;
+		unsigned long nr_reclaimable, nr_writeback;
+
+		nr_zones++;
+
+		zone_dirty_limits(zone, &background_thresh, &dirty_thresh);
+
+		nr_reclaimable = zone_page_state(zone, NR_FILE_DIRTY) +
+			zone_page_state(zone, NR_UNSTABLE_NFS);
+		nr_writeback = zone_page_state(zone, NR_WRITEBACK);
+
+		if (nr_reclaimable + nr_writeback <= background_thresh)
+			continue;
+
+		if (nr_reclaimable > nr_writeback)
+			wakeup_flusher_threads(nr_reclaimable - nr_writeback);
+
+		if (nr_reclaimable + nr_writeback <= dirty_thresh)
+			continue;
+
+		nr_exceeded++;
+	}
+
+	if (nr_zones == nr_exceeded)
+		congestion_wait(BLK_RW_ASYNC, HZ/10);
+}
+
 /*
  * sysctl handler for /proc/sys/vm/dirty_writeback_centisecs
  */
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 4e8985a..1fac154 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1666,6 +1666,9 @@ zonelist_scan:
 			!cpuset_zone_allowed_softwall(zone, gfp_mask))
 				goto try_next_zone;
 
+		if ((gfp_mask & __GFP_WRITE) && !zone_dirty_ok(zone))
+			goto this_zone_full;
+
 		BUILD_BUG_ON(ALLOC_NO_WATERMARKS < NR_WMARK);
 		if (!(alloc_flags & ALLOC_NO_WATERMARKS)) {
 			unsigned long mark;
@@ -1863,6 +1866,22 @@ out:
 	return page;
 }
 
+static struct page *
+__alloc_pages_writeback(gfp_t gfp_mask, unsigned int order,
+			struct zonelist *zonelist, enum zone_type high_zoneidx,
+			nodemask_t *nodemask, int alloc_flags,
+			struct zone *preferred_zone, int migratetype)
+{
+	if (!(gfp_mask & __GFP_WRITE))
+		return NULL;
+
+	try_to_writeback_pages(zonelist, gfp_mask, nodemask);
+
+	return get_page_from_freelist(gfp_mask, nodemask, order, zonelist,
+				      high_zoneidx, alloc_flags,
+				      preferred_zone, migratetype);
+}
+
 #ifdef CONFIG_COMPACTION
 /* Try memory compaction for high-order allocations before reclaim */
 static struct page *
@@ -2135,6 +2154,14 @@ rebalance:
 	if (test_thread_flag(TIF_MEMDIE) && !(gfp_mask & __GFP_NOFAIL))
 		goto nopage;
 
+	/* Try writing back pages if per-zone dirty limits are reached */
+	page = __alloc_pages_writeback(gfp_mask, order, zonelist,
+				       high_zoneidx, nodemask,
+				       alloc_flags, preferred_zone,
+				       migratetype);
+	if (page)
+		goto got_pg;
+
 	/*
 	 * Try direct compaction. The first pass is asynchronous. Subsequent
 	 * attempts after direct reclaim are synchronous
-- 
1.7.6

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