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Date:	Wed, 7 Mar 2012 18:18:53 -0800
From:	"Darrick J. Wong" <djwong@...ibm.com>
To:	Eric Sandeen <sandeen@...hat.com>
Cc:	"Theodore Ts'o" <tytso@....edu>, linux-fsdevel@...r.kernel.org,
	linux-ext4@...r.kernel.org
Subject: Re: [PATCH, RFC] Don't do page stablization if
	!CONFIG_BLKDEV_INTEGRITY

On Wed, Mar 07, 2012 at 04:05:10PM -0800, Darrick J. Wong wrote:
> On Wed, Mar 07, 2012 at 05:54:11PM -0600, Eric Sandeen wrote:
> > On 3/7/12 5:40 PM, Theodore Ts'o wrote:
> > > We've recently discovered a workload at Google where the page
> > > stablization patches (specifically commit 0e499890c1f: ext4: wait for
> > > writeback to complete while making pages writable) resulted in a
> > > **major** performance regression.  As in, kernel threads that were
> > > writing to log files were getting hit by up to 2 seconds stalls, which
> > > very badly hurt a particular application.  Reverting this commit fixed
> > > the performance regression.
> > > 
> > > The main reason for the page stablizatoin patches was for DIF/DIX
> > > support, right?   So I'm wondering if we should just disable the calls
> > > to wait_on_page_writeback if CONFIG_BLKDEV_INTEGRITY is not defined.
> > > i.e., something like this.
> > 
> > Can you devise a non-secret testcase that demonstrates this?
> 
> It sure would be nice if the block device could know if it requires stable
> writeback, and the fs could figure that out.... though iirc there was more to
> my patchset than just these two wait_on_page_writeback() calls.

Ted,

Would something along the lines of the following patch address your concern in
a somewhat more flexible manner?

Provide a BDI flag to indicate that a device requires stable pages during
writeback.  Use the flag to skip wait_on_page_writeback() if we don't have such
a device that needs it.

(Obviously this needs to be refactored a bit...)

Signed-off-by: Darrick J. Wong <djwong@...ibm.com>
---

 block/blk-integrity.c       |    7 +++++++
 fs/buffer.c                 |    2 +-
 fs/ext4/inode.c             |    2 +-
 include/linux/backing-dev.h |    3 +++
 include/linux/fs.h          |    1 +
 include/linux/mm.h          |   13 +------------
 include/linux/pagemap.h     |   14 ++++++++++++++
 mm/filemap.c                |    2 +-
 mm/memory.c                 |   14 ++++++++++++++
 mm/page-writeback.c         |   10 ++++++++++
 10 files changed, 53 insertions(+), 15 deletions(-)

diff --git a/block/blk-integrity.c b/block/blk-integrity.c
index da2a818..f2d51f9 100644
--- a/block/blk-integrity.c
+++ b/block/blk-integrity.c
@@ -420,6 +420,10 @@ int blk_integrity_register(struct gendisk *disk, struct blk_integrity *template)
 	} else
 		bi->name = bi_unsupported_name;
 
+#ifdef CONFIG_BLK_DEV_INTEGRITY
+	disk->queue->backing_dev_info.state |= (1 << BDI_stable_writes);
+#endif
+
 	return 0;
 }
 EXPORT_SYMBOL(blk_integrity_register);
@@ -438,6 +442,9 @@ void blk_integrity_unregister(struct gendisk *disk)
 	if (!disk || !disk->integrity)
 		return;
 
+#ifdef CONFIG_BLK_DEV_INTEGRITY
+	disk->queue->backing_dev_info.state &= ~(1 << BDI_stable_writes);
+#endif
 	bi = disk->integrity;
 
 	kobject_uevent(&bi->kobj, KOBJ_REMOVE);
diff --git a/fs/buffer.c b/fs/buffer.c
index 1a30db7..d994d3d 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -2333,7 +2333,7 @@ int __block_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf,
 		ret = -EAGAIN;
 		goto out_unlock;
 	}
-	wait_on_page_writeback(page);
+	wait_on_stable_page_writeback(page);
 	return 0;
 out_unlock:
 	unlock_page(page);
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index e94ac91..79e6c90 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -4724,7 +4724,7 @@ int ext4_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
 		if (!walk_page_buffers(NULL, page_buffers(page), 0, len, NULL,
 					ext4_bh_unmapped)) {
 			/* Wait so that we don't change page under IO */
-			wait_on_page_writeback(page);
+			wait_on_stable_page_writeback(page);
 			ret = VM_FAULT_LOCKED;
 			goto out;
 		}
diff --git a/include/linux/backing-dev.h b/include/linux/backing-dev.h
index b1038bd..a28fecb 100644
--- a/include/linux/backing-dev.h
+++ b/include/linux/backing-dev.h
@@ -32,6 +32,9 @@ enum bdi_state {
 	BDI_sync_congested,	/* The sync queue is getting full */
 	BDI_registered,		/* bdi_register() was done */
 	BDI_writeback_running,	/* Writeback is in progress */
+#ifdef CONFIG_BLK_DEV_INTEGRITY
+	BDI_stable_writes,	/* Pages must not change during write */
+#endif
 	BDI_unused,		/* Available bits start here */
 };
 
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 69cd5bb..d1eb3ce 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -709,6 +709,7 @@ struct block_device {
 #define PAGECACHE_TAG_TOWRITE	2
 
 int mapping_tagged(struct address_space *mapping, int tag);
+int mapping_needs_stable_writes(struct address_space *as);
 
 /*
  * Might pages of this file be mapped into userspace?
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 17b27cd..a069bcf 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -783,18 +783,7 @@ void page_address_init(void);
 #define PAGE_MAPPING_KSM	2
 #define PAGE_MAPPING_FLAGS	(PAGE_MAPPING_ANON | PAGE_MAPPING_KSM)
 
-extern struct address_space swapper_space;
-static inline struct address_space *page_mapping(struct page *page)
-{
-	struct address_space *mapping = page->mapping;
-
-	VM_BUG_ON(PageSlab(page));
-	if (unlikely(PageSwapCache(page)))
-		mapping = &swapper_space;
-	else if ((unsigned long)mapping & PAGE_MAPPING_ANON)
-		mapping = NULL;
-	return mapping;
-}
+struct address_space *page_mapping(struct page *page);
 
 /* Neutral page->mapping pointer to address_space or anon_vma or other */
 static inline void *page_rmapping(struct page *page)
diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index cfaaa69..0f82b91 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -392,6 +392,20 @@ static inline void wait_on_page_writeback(struct page *page)
 		wait_on_page_bit(page, PG_writeback);
 }
 
+static inline void wait_on_stable_page_writeback(struct page *page)
+{
+#ifdef CONFIG_BLK_DEV_INTEGRITY
+	struct address_space *as;
+
+	as = page_mapping(page);
+	if (!as)
+		return;
+
+	if (mapping_needs_stable_writes(as))
+		wait_on_page_writeback(page);
+#endif
+}
+
 extern void end_page_writeback(struct page *page);
 
 /*
diff --git a/mm/filemap.c b/mm/filemap.c
index b662757..08ffa94 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2361,7 +2361,7 @@ repeat:
 		return NULL;
 	}
 found:
-	wait_on_page_writeback(page);
+	wait_on_stable_page_writeback(page);
 	return page;
 }
 EXPORT_SYMBOL(grab_cache_page_write_begin);
diff --git a/mm/memory.c b/mm/memory.c
index fa2f04e..40288e5 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -67,6 +67,20 @@
 
 #include "internal.h"
 
+extern struct address_space swapper_space;
+struct address_space *page_mapping(struct page *page)
+{
+	struct address_space *mapping = page->mapping;
+
+	VM_BUG_ON(PageSlab(page));
+	if (unlikely(PageSwapCache(page)))
+		mapping = &swapper_space;
+	else if ((unsigned long)mapping & PAGE_MAPPING_ANON)
+		mapping = NULL;
+	return mapping;
+}
+EXPORT_SYMBOL(page_mapping);
+
 #ifndef CONFIG_NEED_MULTIPLE_NODES
 /* use the per-pgdat data instead for discontigmem - mbligh */
 unsigned long max_mapnr;
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 363ba70..dc86f8f 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -2239,3 +2239,13 @@ int mapping_tagged(struct address_space *mapping, int tag)
 	return radix_tree_tagged(&mapping->page_tree, tag);
 }
 EXPORT_SYMBOL(mapping_tagged);
+
+int mapping_needs_stable_writes(struct address_space *as)
+{
+#ifdef CONFIG_BLK_DEV_INTEGRITY
+	if (as->backing_dev_info->state & (1 << BDI_stable_writes))
+		return 1;
+#endif
+	return 0;
+}
+EXPORT_SYMBOL_GPL(mapping_needs_stable_writes);

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