[PATCH 0/2] Reduce system disruption due to kswapd followup

[PATCH 0/2] Reduce system disruption due to kswapd followup

[Mel Gorman]

Further testing of the "Reduce system disruption due to kswapd" discovered
a few problems.  First, as pages were not being swapped, the file LRU was
being scanned faster and clean file pages were being reclaimed resulting
in some cases in larger amounts of read IO to re-read data from disk.
Second, more pages were being written from kswapd context which can
adversly affect IO performance. Lastly, it was observed that PageDirty
pages are not necessarily dirty on all filesystems (buffers can be clean
while PageDirty is set and ->writepage generates no IO) and not all
filesystems set PageWriteback when the page is being written (e.g. ext3).
This disconnect confuses the reclaim stalling logic. This follow-up series
is aimed at these problems.

The tests were based on three kernels

vanilla:	kernel 3.9 as that is what the current mmotm uses as a baseline
mmotm-20130522	is mmotm as of 22nd May with "Reduce system disruption due to
		kswapd" applied on top as per what should be in Andrew's tree
		right now
lessdisrupt-v5r4 is this follow-up series on top of the mmotm kernel

The first test used memcached+memcachetest while some background IO
was in progress as implemented by the parallel IO tests implement in
MM Tests. memcachetest benchmarks how many operations/second memcached
can service. It starts with no background IO on a freshly created ext4
filesystem and then re-runs the test with larger amounts of IO in the
background to roughly simulate a large copy in progress. The expectation
is that the IO should have little or no impact on memcachetest which is
running entirely in memory.

                                             3.9.0                       3.9.0                       3.9.0
                                           vanilla          mm1-mmotm-20130522        mm1-lessdisrupt-v5r4
Ops memcachetest-0M             23117.00 (  0.00%)          23088.00 ( -0.13%)          22815.00 ( -1.31%)
Ops memcachetest-715M           23774.00 (  0.00%)          23504.00 ( -1.14%)          23342.00 ( -1.82%)
Ops memcachetest-2385M           4208.00 (  0.00%)          23740.00 (464.16%)          24138.00 (473.62%)
Ops memcachetest-4055M           4104.00 (  0.00%)          24800.00 (504.29%)          24930.00 (507.46%)
Ops io-duration-0M                  0.00 (  0.00%)              0.00 (  0.00%)              0.00 (  0.00%)
Ops io-duration-715M               12.00 (  0.00%)              7.00 ( 41.67%)              7.00 ( 41.67%)
Ops io-duration-2385M             116.00 (  0.00%)             21.00 ( 81.90%)             21.00 ( 81.90%)
Ops io-duration-4055M             160.00 (  0.00%)             37.00 ( 76.88%)             36.00 ( 77.50%)
Ops swaptotal-0M                    0.00 (  0.00%)              0.00 (  0.00%)              0.00 (  0.00%)
Ops swaptotal-715M             140138.00 (  0.00%)             18.00 ( 99.99%)             18.00 ( 99.99%)
Ops swaptotal-2385M            385682.00 (  0.00%)              0.00 (  0.00%)              0.00 (  0.00%)
Ops swaptotal-4055M            418029.00 (  0.00%)              0.00 (  0.00%)              2.00 (100.00%)
Ops swapin-0M                       0.00 (  0.00%)              0.00 (  0.00%)              0.00 (  0.00%)
Ops swapin-715M                   144.00 (  0.00%)              0.00 (  0.00%)              0.00 (  0.00%)
Ops swapin-2385M               134227.00 (  0.00%)              0.00 (  0.00%)              0.00 (  0.00%)
Ops swapin-4055M               125618.00 (  0.00%)              0.00 (  0.00%)              0.00 (  0.00%)
Ops minorfaults-0M            1536429.00 (  0.00%)        1533759.00 (  0.17%)        1537248.00 ( -0.05%)
Ops minorfaults-715M          1786996.00 (  0.00%)        1606613.00 ( 10.09%)        1610854.00 (  9.86%)
Ops minorfaults-2385M         1757952.00 (  0.00%)        1608201.00 (  8.52%)        1614772.00 (  8.14%)
Ops minorfaults-4055M         1774460.00 (  0.00%)        1620493.00 (  8.68%)        1625930.00 (  8.37%)
Ops majorfaults-0M                  1.00 (  0.00%)              0.00 (  0.00%)              0.00 (  0.00%)
Ops majorfaults-715M              184.00 (  0.00%)            159.00 ( 13.59%)            162.00 ( 11.96%)
Ops majorfaults-2385M           24444.00 (  0.00%)            108.00 ( 99.56%)            151.00 ( 99.38%)
Ops majorfaults-4055M           21357.00 (  0.00%)            218.00 ( 98.98%)            189.00 ( 99.12%)


memcachetest is the transactions/second reported by memcachetest. In
        the vanilla kernel note that performance drops from around
        23K/sec to just over 4K/second when there is 2385M of IO going
        on in the background. With current mmotm, there is no collapse
	in performance and with this follow-up series there is little
	change.

swaptotal is the total amount of swap traffic. With mmotm and the follow-up
	series, the total amount of swapping is much reduced.

                                 3.9.0       3.9.0       3.9.0
                               vanillamm1-mmotm-20130522mm1-lessdisrupt-v5r4
Minor Faults                  11160152    10592704    10620743
Major Faults                     46305         771         788
Swap Ins                        260249           0           0
Swap Outs                       683860          18          20
Direct pages scanned                 0           0         850
Kswapd pages scanned           6046108    18523180     1598979
Kswapd pages reclaimed         1081954     1182759     1093766
Direct pages reclaimed               0           0         800
Kswapd efficiency                  17%          6%         68%
Kswapd velocity               5217.560   16027.810    1382.231
Direct efficiency                 100%        100%         94%
Direct velocity                  0.000       0.000       0.735
Percentage direct scans             0%          0%          0%
Zone normal velocity          5105.086   15217.472     636.579
Zone dma32 velocity            112.473     810.338     746.387
Zone dma velocity                0.000       0.000       0.000
Page writes by reclaim     1929612.00016620834.000   43115.000
Page writes file               1245752    16620816       43095
Page writes anon                683860          18          20
Page reclaim immediate            7484          70         147
Sector Reads                   1130320       94964       97244
Sector Writes                 13508052    11356812    11469072
Page rescued immediate               0           0           0
Slabs scanned                    33536       27648       21120
Direct inode steals                  0           0           0
Kswapd inode steals               8641        1495           0
Kswapd skipped wait                  0           0           0
THP fault alloc                      8           9          39
THP collapse alloc                 508         476         378
THP splits                          24           0           0
THP fault fallback                   0           0           0
THP collapse fail                    0           0           0

There are a number of observations to make here

1. Swap outs are almost eliminated. Swap ins are 0 indicating that the
   pages swapped were really unused anonymous pages. Related to that,
   major faults are much reduced.

2. kswapd efficiency was impacted by the initial series but with these
   follow-up patches, the efficiency is now at 66% indicating that far
   fewer pages were skipped during scanning due to dirty or writeback
   pages.

3. kswapd velocity is reduced indicating that fewer pages are being scanned
   with the follow-up series as kswapd now stalls when the tail of the
   LRU queue is full of unqueued dirty pages. The stall gives flushers a
   chance to catch-up so kswapd can reclaim clean pages when it wakes

4. In light of Zlatko's recent reports about zone scanning imbalances,
   mmtests now reports scanning velocity on a per-zone basis. With mainline,
   you can see that the scanning activity is dominated by the Normal
   zone with over 45 times more scanning in Normal than the DMA32 zone.
   With the series currently in mmotm, the ratio is slightly better but it
   is still the case that the bulk of scanning is in the highest zone. With
   this follow-up series, the ratio of scanning between the Normal and
   DMA32 zone is roughly equal.

5. As Dave Chinner observed, the current patches in mmotm increased the
   number of pages written from kswapd context which is expected to adversly
   impact IO performance. With the follow-up patches, far fewer pages are
   written from kswapd context than the mainline kernel

6. With the series in mmotm, fewer inodes were reclaimed by kswapd. With
   the follow-up series, there is less slab shrinking activity and no inodes
   were reclaimed.

7. Note that "Sectors Read" is drastically reduced implying that the source
   data being used for the IO is not being aggressively discarded due to
   page reclaim skipping over dirty pages and reclaiming clean pages. Note
   that the reducion in reads could also be due to inode data not being
   re-read from disk after a slab shrink.

Overall, the system is getting less kicked in the face due to IO.

 fs/buffer.c                 | 34 ++++++++++++++++++++++++++++++++++
 fs/ext2/inode.c             |  1 +
 fs/ext3/inode.c             |  3 +++
 fs/ext4/inode.c             |  2 ++
 fs/gfs2/aops.c              |  2 ++
 fs/ntfs/aops.c              |  1 +
 fs/ocfs2/aops.c             |  1 +
 fs/xfs/xfs_aops.c           |  1 +
 include/linux/buffer_head.h |  3 +++
 include/linux/fs.h          |  1 +
 mm/vmscan.c                 | 45 ++++++++++++++++++++++++++++++++++++++-------
 11 files changed, 87 insertions(+), 7 deletions(-)

-- 
1.8.1.4

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[PATCH 1/2] mm: vmscan: mm: vmscan: Have kswapd writeback pages based on dirty pages encountered, not priority -fix

[Mel Gorman]

If a zone is marked "reclaim dirty" then kswapd starts writing back pages
but this situation is flagged too easily and flushers are not given the
opportunity to catch up. This patch causes kswapd to only start writing back
pages if all dirty pages scanned at the tail of the LRU are unqueued. If
a zone is flagged as "reclaim dirty", the reclaiming process will stall
to give flushers a chance to clean up. It also renames nr_dirty to
nr_unqueued dirty in shrink_inactive_list() to clarify.

This could be treated as a fix to the patch
mm-vmscan-have-kswapd-writeback-pages-based-on-dirty-pages-encountered-not-priority.patch

Signed-off-by: Mel Gorman <mgorman@suse.de>
---
 mm/vmscan.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index b1b38ad..f3315c6 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1316,7 +1316,7 @@ shrink_inactive_list(unsigned long nr_to_scan, struct lruvec *lruvec,
 	unsigned long nr_scanned;
 	unsigned long nr_reclaimed = 0;
 	unsigned long nr_taken;
-	unsigned long nr_dirty = 0;
+	unsigned long nr_unqueued_dirty = 0;
 	unsigned long nr_writeback = 0;
 	isolate_mode_t isolate_mode = 0;
 	int file = is_file_lru(lru);
@@ -1359,7 +1359,7 @@ shrink_inactive_list(unsigned long nr_to_scan, struct lruvec *lruvec,
 		return 0;
 
 	nr_reclaimed = shrink_page_list(&page_list, zone, sc, TTU_UNMAP,
-					&nr_dirty, &nr_writeback, false);
+				&nr_unqueued_dirty, &nr_writeback, false);
 
 	spin_lock_irq(&zone->lru_lock);
 
@@ -1414,11 +1414,13 @@ shrink_inactive_list(unsigned long nr_to_scan, struct lruvec *lruvec,
 	/*
 	 * Similarly, if many dirty pages are encountered that are not
 	 * currently being written then flag that kswapd should start
-	 * writing back pages.
+	 * writing back pages and stall to give a chance for flushers
+	 * to catch up.
 	 */
-	if (global_reclaim(sc) && nr_dirty &&
-			nr_dirty >= (nr_taken >> (DEF_PRIORITY - sc->priority)))
+	if (global_reclaim(sc) && nr_unqueued_dirty == nr_taken) {
+		congestion_wait(BLK_RW_ASYNC, HZ/10);
 		zone_set_flag(zone, ZONE_TAIL_LRU_DIRTY);
+	}
 
 	trace_mm_vmscan_lru_shrink_inactive(zone->zone_pgdat->node_id,
 		zone_idx(zone),
-- 
1.8.1.4

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[PATCH 2/2] mm: vmscan: Take page buffers dirty and locked state into account

[Mel Gorman]

Page reclaim keeps track of dirty and under writeback pages and uses it to
determine if wait_iff_congested() should stall or if kswapd should begin
writing back pages. This fails to account for buffer pages that can be
under writeback but not PageWriteback which is the case for filesystems
like ext3. Furthermore, PageDirty buffer pages can have all the buffers
clean and writepage does no IO so it should not be accounted as congested.

This patch adds an address_space operation that filesystems may
optionally use to check if a page is really dirty or really under
writeback. An implementation is provided for filesystems that use
buffer_heads. By default, the page flags are obeyed.

Credit goes to Jan Kara for identifying that the page flags alone are
not sufficient for ext3 and sanity checking a number of ideas on how
the problem could be addressed.

Signed-off-by: Mel Gorman <mgorman@suse.de>
---
 fs/buffer.c                 | 34 ++++++++++++++++++++++++++++++++++
 fs/ext2/inode.c             |  1 +
 fs/ext3/inode.c             |  3 +++
 fs/ext4/inode.c             |  2 ++
 fs/gfs2/aops.c              |  2 ++
 fs/ntfs/aops.c              |  1 +
 fs/ocfs2/aops.c             |  1 +
 fs/xfs/xfs_aops.c           |  1 +
 include/linux/buffer_head.h |  3 +++
 include/linux/fs.h          |  1 +
 mm/vmscan.c                 | 33 +++++++++++++++++++++++++++++++--
 11 files changed, 80 insertions(+), 2 deletions(-)

diff --git a/fs/buffer.c b/fs/buffer.c
index 1aa0836..4247aa9 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -91,6 +91,40 @@ void unlock_buffer(struct buffer_head *bh)
 EXPORT_SYMBOL(unlock_buffer);
 
 /*
+ * Returns if the page has dirty or writeback buffers. If all the buffers
+ * are unlocked and clean then the PageDirty information is stale. If
+ * any of the pages are locked, it is assumed they are locked for IO.
+ */
+void buffer_check_dirty_writeback(struct page *page,
+				     bool *dirty, bool *writeback)
+{
+	struct buffer_head *head, *bh;
+	*dirty = false;
+	*writeback = false;
+
+	BUG_ON(!PageLocked(page));
+
+	if (!page_has_buffers(page))
+		return;
+
+	if (PageWriteback(page))
+		*writeback = true;
+
+	head = page_buffers(page);
+	bh = head;
+	do {
+		if (buffer_locked(bh))
+			*writeback = true;
+
+		if (buffer_dirty(bh))
+			*dirty = true;
+
+		bh = bh->b_this_page;
+	} while (bh != head);
+}
+EXPORT_SYMBOL(buffer_check_dirty_writeback);
+
+/*
  * Block until a buffer comes unlocked.  This doesn't stop it
  * from becoming locked again - you have to lock it yourself
  * if you want to preserve its state.
diff --git a/fs/ext2/inode.c b/fs/ext2/inode.c
index 0a87bb1..2fc3593 100644
--- a/fs/ext2/inode.c
+++ b/fs/ext2/inode.c
@@ -880,6 +880,7 @@ const struct address_space_operations ext2_aops = {
 	.writepages		= ext2_writepages,
 	.migratepage		= buffer_migrate_page,
 	.is_partially_uptodate	= block_is_partially_uptodate,
+	.is_dirty_writeback	= buffer_check_dirty_writeback,
 	.error_remove_page	= generic_error_remove_page,
 };
 
diff --git a/fs/ext3/inode.c b/fs/ext3/inode.c
index 23c7128..14494fc 100644
--- a/fs/ext3/inode.c
+++ b/fs/ext3/inode.c
@@ -1984,6 +1984,7 @@ static const struct address_space_operations ext3_ordered_aops = {
 	.direct_IO		= ext3_direct_IO,
 	.migratepage		= buffer_migrate_page,
 	.is_partially_uptodate  = block_is_partially_uptodate,
+	.is_dirty_writeback	= buffer_check_dirty_writeback,
 	.error_remove_page	= generic_error_remove_page,
 };
 
@@ -1999,6 +2000,7 @@ static const struct address_space_operations ext3_writeback_aops = {
 	.direct_IO		= ext3_direct_IO,
 	.migratepage		= buffer_migrate_page,
 	.is_partially_uptodate  = block_is_partially_uptodate,
+	.is_dirty_writeback	= buffer_check_dirty_writeback,
 	.error_remove_page	= generic_error_remove_page,
 };
 
@@ -2013,6 +2015,7 @@ static const struct address_space_operations ext3_journalled_aops = {
 	.invalidatepage		= ext3_invalidatepage,
 	.releasepage		= ext3_releasepage,
 	.is_partially_uptodate  = block_is_partially_uptodate,
+	.is_dirty_writeback	= buffer_check_dirty_writeback,
 	.error_remove_page	= generic_error_remove_page,
 };
 
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 0723774..7af746a 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -3309,6 +3309,7 @@ static const struct address_space_operations ext4_aops = {
 	.direct_IO		= ext4_direct_IO,
 	.migratepage		= buffer_migrate_page,
 	.is_partially_uptodate  = block_is_partially_uptodate,
+	.is_dirty_writeback	= buffer_check_dirty_writeback,
 	.error_remove_page	= generic_error_remove_page,
 };
 
@@ -3340,6 +3341,7 @@ static const struct address_space_operations ext4_da_aops = {
 	.direct_IO		= ext4_direct_IO,
 	.migratepage		= buffer_migrate_page,
 	.is_partially_uptodate  = block_is_partially_uptodate,
+	.is_dirty_writeback	= buffer_check_dirty_writeback,
 	.error_remove_page	= generic_error_remove_page,
 };
 
diff --git a/fs/gfs2/aops.c b/fs/gfs2/aops.c
index 0bad69e..027b8ea 100644
--- a/fs/gfs2/aops.c
+++ b/fs/gfs2/aops.c
@@ -1112,6 +1112,7 @@ static const struct address_space_operations gfs2_writeback_aops = {
 	.direct_IO = gfs2_direct_IO,
 	.migratepage = buffer_migrate_page,
 	.is_partially_uptodate = block_is_partially_uptodate,
+	.is_dirty_writeback = buffer_check_dirty_writeback,
 	.error_remove_page = generic_error_remove_page,
 };
 
@@ -1129,6 +1130,7 @@ static const struct address_space_operations gfs2_ordered_aops = {
 	.direct_IO = gfs2_direct_IO,
 	.migratepage = buffer_migrate_page,
 	.is_partially_uptodate = block_is_partially_uptodate,
+	.is_dirty_writeback = buffer_check_dirty_writeback,
 	.error_remove_page = generic_error_remove_page,
 };
 
diff --git a/fs/ntfs/aops.c b/fs/ntfs/aops.c
index fa9c05f..eb85ac1 100644
--- a/fs/ntfs/aops.c
+++ b/fs/ntfs/aops.c
@@ -1549,6 +1549,7 @@ const struct address_space_operations ntfs_aops = {
 	.migratepage	= buffer_migrate_page,	/* Move a page cache page from
 						   one physical page to an
 						   other. */
+	.is_dirty_writeback = buffer_check_dirty_writeback,
 	.error_remove_page = generic_error_remove_page,
 };
 
diff --git a/fs/ocfs2/aops.c b/fs/ocfs2/aops.c
index 20dfec7..191af11 100644
--- a/fs/ocfs2/aops.c
+++ b/fs/ocfs2/aops.c
@@ -2096,5 +2096,6 @@ const struct address_space_operations ocfs2_aops = {
 	.releasepage		= ocfs2_releasepage,
 	.migratepage		= buffer_migrate_page,
 	.is_partially_uptodate	= block_is_partially_uptodate,
+	.is_dirty_writeback	= buffer_check_dirty_writeback,
 	.error_remove_page	= generic_error_remove_page,
 };
diff --git a/fs/xfs/xfs_aops.c b/fs/xfs/xfs_aops.c
index f64ee71..1aada1c 100644
--- a/fs/xfs/xfs_aops.c
+++ b/fs/xfs/xfs_aops.c
@@ -1656,5 +1656,6 @@ const struct address_space_operations xfs_address_space_operations = {
 	.direct_IO		= xfs_vm_direct_IO,
 	.migratepage		= buffer_migrate_page,
 	.is_partially_uptodate  = block_is_partially_uptodate,
+	.is_dirty_writeback	= buffer_check_dirty_writeback,
 	.error_remove_page	= generic_error_remove_page,
 };
diff --git a/include/linux/buffer_head.h b/include/linux/buffer_head.h
index 6d9f5a2..d458880 100644
--- a/include/linux/buffer_head.h
+++ b/include/linux/buffer_head.h
@@ -139,6 +139,9 @@ BUFFER_FNS(Prio, prio)
 	})
 #define page_has_buffers(page)	PagePrivate(page)
 
+void buffer_check_dirty_writeback(struct page *page,
+				     bool *dirty, bool *writeback);
+
 /*
  * Declarations
  */
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 0a9a6766..96f857f 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -380,6 +380,7 @@ struct address_space_operations {
 	int (*launder_page) (struct page *);
 	int (*is_partially_uptodate) (struct page *, read_descriptor_t *,
 					unsigned long);
+	void (*is_dirty_writeback) (struct page *, bool *, bool *);
 	int (*error_remove_page)(struct address_space *, struct page *);
 
 	/* swapfile support */
diff --git a/mm/vmscan.c b/mm/vmscan.c
index f3315c6..d9213d8 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -669,6 +669,25 @@ static enum page_references page_check_references(struct page *page,
 	return PAGEREF_RECLAIM;
 }
 
+/* Check if a page is dirty or under writeback */
+static void page_check_dirty_writeback(struct page *page,
+				       bool *dirty, bool *writeback)
+{
+	struct address_space *mapping;
+
+	/* By default assume that the page flags are accurate */
+	*dirty = PageDirty(page);
+	*writeback = PageWriteback(page);
+
+	/* Verify dirty/writeback state if the filesystem supports it */
+	if (!page_has_private(page))
+		return;
+
+	mapping = page_mapping(page);
+	if (mapping && mapping->a_ops->is_dirty_writeback)
+		mapping->a_ops->is_dirty_writeback(page, dirty, writeback);
+}
+
 /*
  * shrink_page_list() returns the number of reclaimed pages
  */
@@ -839,9 +858,19 @@ static unsigned long shrink_page_list(struct list_head *page_list,
 		}
 
 		if (PageDirty(page)) {
-			nr_dirty++;
+			bool dirty, writeback;
+
+			/*
+			 * The number of dirty pages determines if a zone is
+			 * marked zone_is_reclaim_congested which affects
+			 * wait_iff_congested. The number of unqueued dirty
+			 * pages affects if kswapd will start writing pages.
+			 */
+			page_check_dirty_writeback(page, &dirty, &writeback);
+			if (dirty || writeback)
+				nr_dirty++;
 
-			if (!PageWriteback(page))
+			if (dirty && !writeback)
 				nr_unqueued_dirty++;
 
 			/*
-- 
1.8.1.4

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Re: [PATCH 2/2] mm: vmscan: Take page buffers dirty and locked state into account

[Jan Kara]

On Thu 23-05-13 10:26:27, Mel Gorman wrote:
> Page reclaim keeps track of dirty and under writeback pages and uses it to
> determine if wait_iff_congested() should stall or if kswapd should begin
> writing back pages. This fails to account for buffer pages that can be
> under writeback but not PageWriteback which is the case for filesystems
> like ext3. Furthermore, PageDirty buffer pages can have all the buffers
> clean and writepage does no IO so it should not be accounted as congested.
> 
> This patch adds an address_space operation that filesystems may
> optionally use to check if a page is really dirty or really under
> writeback. An implementation is provided for filesystems that use
> buffer_heads. By default, the page flags are obeyed.
> 
> Credit goes to Jan Kara for identifying that the page flags alone are
> not sufficient for ext3 and sanity checking a number of ideas on how
> the problem could be addressed.
> 
> Signed-off-by: Mel Gorman <mgorman@suse.de>
> ---
>  fs/buffer.c                 | 34 ++++++++++++++++++++++++++++++++++
>  fs/ext2/inode.c             |  1 +
>  fs/ext3/inode.c             |  3 +++
>  fs/ext4/inode.c             |  2 ++
>  fs/gfs2/aops.c              |  2 ++
>  fs/ntfs/aops.c              |  1 +
>  fs/ocfs2/aops.c             |  1 +
>  fs/xfs/xfs_aops.c           |  1 +
>  include/linux/buffer_head.h |  3 +++
>  include/linux/fs.h          |  1 +
>  mm/vmscan.c                 | 33 +++++++++++++++++++++++++++++++--
>  11 files changed, 80 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/buffer.c b/fs/buffer.c
> index 1aa0836..4247aa9 100644
> --- a/fs/buffer.c
> +++ b/fs/buffer.c
> @@ -91,6 +91,40 @@ void unlock_buffer(struct buffer_head *bh)
>  EXPORT_SYMBOL(unlock_buffer);
>  
>  /*
> + * Returns if the page has dirty or writeback buffers. If all the buffers
> + * are unlocked and clean then the PageDirty information is stale. If
> + * any of the pages are locked, it is assumed they are locked for IO.
> + */
> +void buffer_check_dirty_writeback(struct page *page,
> +				     bool *dirty, bool *writeback)
> +{
> +	struct buffer_head *head, *bh;
> +	*dirty = false;
> +	*writeback = false;
> +
> +	BUG_ON(!PageLocked(page));
> +
> +	if (!page_has_buffers(page))
> +		return;
> +
> +	if (PageWriteback(page))
> +		*writeback = true;
> +
> +	head = page_buffers(page);
> +	bh = head;
> +	do {
> +		if (buffer_locked(bh))
> +			*writeback = true;
> +
> +		if (buffer_dirty(bh))
> +			*dirty = true;
> +
> +		bh = bh->b_this_page;
> +	} while (bh != head);
> +}
> +EXPORT_SYMBOL(buffer_check_dirty_writeback);
> +
> +/*
>   * Block until a buffer comes unlocked.  This doesn't stop it
>   * from becoming locked again - you have to lock it yourself
>   * if you want to preserve its state.
> diff --git a/fs/ext2/inode.c b/fs/ext2/inode.c
> index 0a87bb1..2fc3593 100644
> --- a/fs/ext2/inode.c
> +++ b/fs/ext2/inode.c
> @@ -880,6 +880,7 @@ const struct address_space_operations ext2_aops = {
>  	.writepages		= ext2_writepages,
>  	.migratepage		= buffer_migrate_page,
>  	.is_partially_uptodate	= block_is_partially_uptodate,
> +	.is_dirty_writeback	= buffer_check_dirty_writeback,
>  	.error_remove_page	= generic_error_remove_page,
>  };
  Hum, actually from what I know, it should be enough to set
.is_dirty_writeback to buffer_check_dirty_writeback() only for
ext3_ordered_aops and maybe def_blk_aops (fs/block_dev.c). I also realized
that data=journal mode of ext3 & ext4 also needs a special treatment but
there we have to have a special function (likely provided by jbd/jbd2). But
this mode isn't used very much so it's not pressing to fix that.

Also I was thinking about how does this work NFS? It's page state logic is
more complex with page going from PageDirty -> PageWriteback -> Unstable ->
Clean. Unstable is a state where the page appears as clean to MM but it
still cannot be reclaimed (we are waiting for the server to write the
page). You need an inode wide commit operation to transform pages from
Unstable to Clean state.
  
I guess it would be worth testing this - something like your largedd test
but over NFS.

								Honza

> diff --git a/fs/ext3/inode.c b/fs/ext3/inode.c
> index 23c7128..14494fc 100644
> --- a/fs/ext3/inode.c
> +++ b/fs/ext3/inode.c
> @@ -1984,6 +1984,7 @@ static const struct address_space_operations ext3_ordered_aops = {
>  	.direct_IO		= ext3_direct_IO,
>  	.migratepage		= buffer_migrate_page,
>  	.is_partially_uptodate  = block_is_partially_uptodate,
> +	.is_dirty_writeback	= buffer_check_dirty_writeback,
>  	.error_remove_page	= generic_error_remove_page,
>  };
>  
> @@ -1999,6 +2000,7 @@ static const struct address_space_operations ext3_writeback_aops = {
>  	.direct_IO		= ext3_direct_IO,
>  	.migratepage		= buffer_migrate_page,
>  	.is_partially_uptodate  = block_is_partially_uptodate,
> +	.is_dirty_writeback	= buffer_check_dirty_writeback,
>  	.error_remove_page	= generic_error_remove_page,
>  };
>  
> @@ -2013,6 +2015,7 @@ static const struct address_space_operations ext3_journalled_aops = {
>  	.invalidatepage		= ext3_invalidatepage,
>  	.releasepage		= ext3_releasepage,
>  	.is_partially_uptodate  = block_is_partially_uptodate,
> +	.is_dirty_writeback	= buffer_check_dirty_writeback,
>  	.error_remove_page	= generic_error_remove_page,
>  };
>  
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index 0723774..7af746a 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -3309,6 +3309,7 @@ static const struct address_space_operations ext4_aops = {
>  	.direct_IO		= ext4_direct_IO,
>  	.migratepage		= buffer_migrate_page,
>  	.is_partially_uptodate  = block_is_partially_uptodate,
> +	.is_dirty_writeback	= buffer_check_dirty_writeback,
>  	.error_remove_page	= generic_error_remove_page,
>  };
>  
> @@ -3340,6 +3341,7 @@ static const struct address_space_operations ext4_da_aops = {
>  	.direct_IO		= ext4_direct_IO,
>  	.migratepage		= buffer_migrate_page,
>  	.is_partially_uptodate  = block_is_partially_uptodate,
> +	.is_dirty_writeback	= buffer_check_dirty_writeback,
>  	.error_remove_page	= generic_error_remove_page,
>  };
>  
> diff --git a/fs/gfs2/aops.c b/fs/gfs2/aops.c
> index 0bad69e..027b8ea 100644
> --- a/fs/gfs2/aops.c
> +++ b/fs/gfs2/aops.c
> @@ -1112,6 +1112,7 @@ static const struct address_space_operations gfs2_writeback_aops = {
>  	.direct_IO = gfs2_direct_IO,
>  	.migratepage = buffer_migrate_page,
>  	.is_partially_uptodate = block_is_partially_uptodate,
> +	.is_dirty_writeback = buffer_check_dirty_writeback,
>  	.error_remove_page = generic_error_remove_page,
>  };
>  
> @@ -1129,6 +1130,7 @@ static const struct address_space_operations gfs2_ordered_aops = {
>  	.direct_IO = gfs2_direct_IO,
>  	.migratepage = buffer_migrate_page,
>  	.is_partially_uptodate = block_is_partially_uptodate,
> +	.is_dirty_writeback = buffer_check_dirty_writeback,
>  	.error_remove_page = generic_error_remove_page,
>  };
>  
> diff --git a/fs/ntfs/aops.c b/fs/ntfs/aops.c
> index fa9c05f..eb85ac1 100644
> --- a/fs/ntfs/aops.c
> +++ b/fs/ntfs/aops.c
> @@ -1549,6 +1549,7 @@ const struct address_space_operations ntfs_aops = {
>  	.migratepage	= buffer_migrate_page,	/* Move a page cache page from
>  						   one physical page to an
>  						   other. */
> +	.is_dirty_writeback = buffer_check_dirty_writeback,
>  	.error_remove_page = generic_error_remove_page,
>  };
>  
> diff --git a/fs/ocfs2/aops.c b/fs/ocfs2/aops.c
> index 20dfec7..191af11 100644
> --- a/fs/ocfs2/aops.c
> +++ b/fs/ocfs2/aops.c
> @@ -2096,5 +2096,6 @@ const struct address_space_operations ocfs2_aops = {
>  	.releasepage		= ocfs2_releasepage,
>  	.migratepage		= buffer_migrate_page,
>  	.is_partially_uptodate	= block_is_partially_uptodate,
> +	.is_dirty_writeback	= buffer_check_dirty_writeback,
>  	.error_remove_page	= generic_error_remove_page,
>  };
> diff --git a/fs/xfs/xfs_aops.c b/fs/xfs/xfs_aops.c
> index f64ee71..1aada1c 100644
> --- a/fs/xfs/xfs_aops.c
> +++ b/fs/xfs/xfs_aops.c
> @@ -1656,5 +1656,6 @@ const struct address_space_operations xfs_address_space_operations = {
>  	.direct_IO		= xfs_vm_direct_IO,
>  	.migratepage		= buffer_migrate_page,
>  	.is_partially_uptodate  = block_is_partially_uptodate,
> +	.is_dirty_writeback	= buffer_check_dirty_writeback,
>  	.error_remove_page	= generic_error_remove_page,
>  };
> diff --git a/include/linux/buffer_head.h b/include/linux/buffer_head.h
> index 6d9f5a2..d458880 100644
> --- a/include/linux/buffer_head.h
> +++ b/include/linux/buffer_head.h
> @@ -139,6 +139,9 @@ BUFFER_FNS(Prio, prio)
>  	})
>  #define page_has_buffers(page)	PagePrivate(page)
>  
> +void buffer_check_dirty_writeback(struct page *page,
> +				     bool *dirty, bool *writeback);
> +
>  /*
>   * Declarations
>   */
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 0a9a6766..96f857f 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -380,6 +380,7 @@ struct address_space_operations {
>  	int (*launder_page) (struct page *);
>  	int (*is_partially_uptodate) (struct page *, read_descriptor_t *,
>  					unsigned long);
> +	void (*is_dirty_writeback) (struct page *, bool *, bool *);
>  	int (*error_remove_page)(struct address_space *, struct page *);
>  
>  	/* swapfile support */
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index f3315c6..d9213d8 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -669,6 +669,25 @@ static enum page_references page_check_references(struct page *page,
>  	return PAGEREF_RECLAIM;
>  }
>  
> +/* Check if a page is dirty or under writeback */
> +static void page_check_dirty_writeback(struct page *page,
> +				       bool *dirty, bool *writeback)
> +{
> +	struct address_space *mapping;
> +
> +	/* By default assume that the page flags are accurate */
> +	*dirty = PageDirty(page);
> +	*writeback = PageWriteback(page);
> +
> +	/* Verify dirty/writeback state if the filesystem supports it */
> +	if (!page_has_private(page))
> +		return;
> +
> +	mapping = page_mapping(page);
> +	if (mapping && mapping->a_ops->is_dirty_writeback)
> +		mapping->a_ops->is_dirty_writeback(page, dirty, writeback);
> +}
> +
>  /*
>   * shrink_page_list() returns the number of reclaimed pages
>   */
> @@ -839,9 +858,19 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>  		}
>  
>  		if (PageDirty(page)) {
> -			nr_dirty++;
> +			bool dirty, writeback;
> +
> +			/*
> +			 * The number of dirty pages determines if a zone is
> +			 * marked zone_is_reclaim_congested which affects
> +			 * wait_iff_congested. The number of unqueued dirty
> +			 * pages affects if kswapd will start writing pages.
> +			 */
> +			page_check_dirty_writeback(page, &dirty, &writeback);
> +			if (dirty || writeback)
> +				nr_dirty++;
>  
> -			if (!PageWriteback(page))
> +			if (dirty && !writeback)
>  				nr_unqueued_dirty++;
>  
>  			/*
> -- 
> 1.8.1.4
> 
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR

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Re: [PATCH 2/2] mm: vmscan: Take page buffers dirty and locked state into account

[Mel Gorman]

On Thu, May 23, 2013 at 11:53:15AM +0200, Jan Kara wrote:
> On Thu 23-05-13 10:26:27, Mel Gorman wrote:
> > Page reclaim keeps track of dirty and under writeback pages and uses it to
> > determine if wait_iff_congested() should stall or if kswapd should begin
> > writing back pages. This fails to account for buffer pages that can be
> > under writeback but not PageWriteback which is the case for filesystems
> > like ext3. Furthermore, PageDirty buffer pages can have all the buffers
> > clean and writepage does no IO so it should not be accounted as congested.
> > 
> > This patch adds an address_space operation that filesystems may
> > optionally use to check if a page is really dirty or really under
> > writeback. An implementation is provided for filesystems that use
> > buffer_heads. By default, the page flags are obeyed.
> > 
> > Credit goes to Jan Kara for identifying that the page flags alone are
> > not sufficient for ext3 and sanity checking a number of ideas on how
> > the problem could be addressed.
> > 
> > Signed-off-by: Mel Gorman <mgorman@suse.de>
> > ---
> >  fs/buffer.c                 | 34 ++++++++++++++++++++++++++++++++++
> >  fs/ext2/inode.c             |  1 +
> >  fs/ext3/inode.c             |  3 +++
> >  fs/ext4/inode.c             |  2 ++
> >  fs/gfs2/aops.c              |  2 ++
> >  fs/ntfs/aops.c              |  1 +
> >  fs/ocfs2/aops.c             |  1 +
> >  fs/xfs/xfs_aops.c           |  1 +
> >  include/linux/buffer_head.h |  3 +++
> >  include/linux/fs.h          |  1 +
> >  mm/vmscan.c                 | 33 +++++++++++++++++++++++++++++++--
> >  11 files changed, 80 insertions(+), 2 deletions(-)
> > 
> > diff --git a/fs/buffer.c b/fs/buffer.c
> > index 1aa0836..4247aa9 100644
> > --- a/fs/buffer.c
> > +++ b/fs/buffer.c
> > @@ -91,6 +91,40 @@ void unlock_buffer(struct buffer_head *bh)
> >  EXPORT_SYMBOL(unlock_buffer);
> >  
> >  /*
> > + * Returns if the page has dirty or writeback buffers. If all the buffers
> > + * are unlocked and clean then the PageDirty information is stale. If
> > + * any of the pages are locked, it is assumed they are locked for IO.
> > + */
> > +void buffer_check_dirty_writeback(struct page *page,
> > +				     bool *dirty, bool *writeback)
> > +{
> > +	struct buffer_head *head, *bh;
> > +	*dirty = false;
> > +	*writeback = false;
> > +
> > +	BUG_ON(!PageLocked(page));
> > +
> > +	if (!page_has_buffers(page))
> > +		return;
> > +
> > +	if (PageWriteback(page))
> > +		*writeback = true;
> > +
> > +	head = page_buffers(page);
> > +	bh = head;
> > +	do {
> > +		if (buffer_locked(bh))
> > +			*writeback = true;
> > +
> > +		if (buffer_dirty(bh))
> > +			*dirty = true;
> > +
> > +		bh = bh->b_this_page;
> > +	} while (bh != head);
> > +}
> > +EXPORT_SYMBOL(buffer_check_dirty_writeback);
> > +
> > +/*
> >   * Block until a buffer comes unlocked.  This doesn't stop it
> >   * from becoming locked again - you have to lock it yourself
> >   * if you want to preserve its state.
> > diff --git a/fs/ext2/inode.c b/fs/ext2/inode.c
> > index 0a87bb1..2fc3593 100644
> > --- a/fs/ext2/inode.c
> > +++ b/fs/ext2/inode.c
> > @@ -880,6 +880,7 @@ const struct address_space_operations ext2_aops = {
> >  	.writepages		= ext2_writepages,
> >  	.migratepage		= buffer_migrate_page,
> >  	.is_partially_uptodate	= block_is_partially_uptodate,
> > +	.is_dirty_writeback	= buffer_check_dirty_writeback,
> >  	.error_remove_page	= generic_error_remove_page,
> >  };
>
>   Hum, actually from what I know, it should be enough to set
> .is_dirty_writeback to buffer_check_dirty_writeback() only for
> ext3_ordered_aops and maybe def_blk_aops (fs/block_dev.c).

Hmm, ok. I had thought that even where the generic write pages were used
that set PageWriteback that it should still benefit from checking if the
buffers were clean. I'll back it out.

I'll add it to def_blk_aops, thanks for pointing that out.

> I also realized
> that data=journal mode of ext3 & ext4 also needs a special treatment but
> there we have to have a special function (likely provided by jbd/jbd2). But
> this mode isn't used very much so it's not pressing to fix that.
> 

And thanks for catching that

> Also I was thinking about how does this work NFS? It's page state logic is
> more complex with page going from PageDirty -> PageWriteback -> Unstable ->
> Clean. Unstable is a state where the page appears as clean to MM but it
> still cannot be reclaimed (we are waiting for the server to write the
> page). You need an inode wide commit operation to transform pages from
> Unstable to Clean state.
>   

I expect they'll be skipped and not accounted for because try_to_release_page
will fail. The pages will move to the active list and do another cycle
through the LRU. If there a lot of these pages then kswapd usage may get
high as it'll not stall. It'll need additional help.

That said, I also notice now that the PageWriteback check in the wrong
place. Pages have their dirty flag cleared under the lock before queueing
for IO until they are either redirtied or under writeback but the accounting
is within a PageDirty check. That needs fixing.

> I guess it would be worth testing this - something like your largedd test
> but over NFS.
> 

I will add it.

-- 
Mel Gorman
SUSE Labs

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