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Date: Wed, 5 May 2010 10:40:56 +0200
From: Jens Axboe <jens.axboe@...cle.com>
To: Dmitry Monakhov <dmonakhov@...nvz.org>
Cc: linux-ext4@...r.kernel.org, chris.mason@...cle.com
Subject: Re: [Bug 15906] serious performance regression in "umount" on ext4
over LVM
On Wed, May 05 2010, Dmitry Monakhov wrote:
> bugzilla-daemon@...zilla.kernel.org writes:
>
> Hi Jens,
>
> Just FYI, we have found a regression which was caused by your famous
> writeback patch 03ba3782e8dcc5b0e1efe440d33084f066e38cae
> I'm not allowed to add you to CC in BZ, that's why i wrote this mail.
You need to use axboe@...nel.dk as that is what I use there.
> Before the patch __sync_filesystem() called writeback_single_inode()
> directly, but now it is called indirectly (from flush-X:X task)
> which require a super_block in question to be pinned.
> But this is impossible to pin this SB on umount because we already
> hold s_umount sem for write, so effectively we already pinned that SB.
> So my proposal is to treat umount similar to WB_SYNC_ALL, and skip
> pining stage.
Hmm I see, yes that is a bug. How about adding a WB_SYNC_NONE_PINNED or
something to that effect, which acts like WB_SYNC_NONE but the caller is
required to hold the s_umount already (like WB_SYNC_ALL).
Something like the below. It compiles, but not tested.
diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 4b37f7c..4327465 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -245,19 +245,20 @@ static void bdi_sync_writeback(struct backing_dev_info *bdi,
* @bdi: the backing device to write from
* @sb: write inodes from this super_block
* @nr_pages: the number of pages to write
+ * @sb_locked: caller already holds sb umount sem.
*
* Description:
* This does WB_SYNC_NONE opportunistic writeback. The IO is only
* started when this function returns, we make no guarentees on
- * completion. Caller need not hold sb s_umount semaphore.
+ * completion. Caller specifies whether sb umount sem is held already or not.
*
*/
void bdi_start_writeback(struct backing_dev_info *bdi, struct super_block *sb,
- long nr_pages)
+ long nr_pages, int sb_locked)
{
struct wb_writeback_args args = {
.sb = sb,
- .sync_mode = WB_SYNC_NONE,
+ .sync_mode = sb_locked ? WB_SYNC_NONE_PIN : WB_SYNC_NONE,
.nr_pages = nr_pages,
.range_cyclic = 1,
};
@@ -577,7 +578,8 @@ static enum sb_pin_state pin_sb_for_writeback(struct writeback_control *wbc,
/*
* Caller must already hold the ref for this
*/
- if (wbc->sync_mode == WB_SYNC_ALL) {
+ if (wbc->sync_mode == WB_SYNC_ALL ||
+ wbc->sync_mode == WB_SYNC_NONE_PIN) {
WARN_ON(!rwsem_is_locked(&sb->s_umount));
return SB_NOT_PINNED;
}
@@ -1183,6 +1185,18 @@ static void wait_sb_inodes(struct super_block *sb)
iput(old_inode);
}
+static void __writeback_inodes_sb(struct super_block *sb, int sb_locked)
+{
+ unsigned long nr_dirty = global_page_state(NR_FILE_DIRTY);
+ unsigned long nr_unstable = global_page_state(NR_UNSTABLE_NFS);
+ long nr_to_write;
+
+ nr_to_write = nr_dirty + nr_unstable +
+ (inodes_stat.nr_inodes - inodes_stat.nr_unused);
+
+ bdi_start_writeback(sb->s_bdi, sb, nr_to_write, sb_locked);
+}
+
/**
* writeback_inodes_sb - writeback dirty inodes from given super_block
* @sb: the superblock
@@ -1194,18 +1208,23 @@ static void wait_sb_inodes(struct super_block *sb)
*/
void writeback_inodes_sb(struct super_block *sb)
{
- unsigned long nr_dirty = global_page_state(NR_FILE_DIRTY);
- unsigned long nr_unstable = global_page_state(NR_UNSTABLE_NFS);
- long nr_to_write;
-
- nr_to_write = nr_dirty + nr_unstable +
- (inodes_stat.nr_inodes - inodes_stat.nr_unused);
-
- bdi_start_writeback(sb->s_bdi, sb, nr_to_write);
+ __writeback_inodes_sb(sb, 0);
}
EXPORT_SYMBOL(writeback_inodes_sb);
/**
+ * writeback_inodes_sb_locked - writeback dirty inodes from given super_block
+ * @sb: the superblock
+ *
+ * Like writeback_inodes_sb(), except the caller already holds the
+ * sb umount sem.
+ */
+void writeback_inodes_sb_locked(struct super_block *sb)
+{
+ __writeback_inodes_sb(sb, 1);
+}
+
+/**
* writeback_inodes_sb_if_idle - start writeback if none underway
* @sb: the superblock
*
diff --git a/fs/sync.c b/fs/sync.c
index 92b2281..de6a441 100644
--- a/fs/sync.c
+++ b/fs/sync.c
@@ -42,7 +42,7 @@ static int __sync_filesystem(struct super_block *sb, int wait)
if (wait)
sync_inodes_sb(sb);
else
- writeback_inodes_sb(sb);
+ writeback_inodes_sb_locked(sb);
if (sb->s_op->sync_fs)
sb->s_op->sync_fs(sb, wait);
diff --git a/include/linux/backing-dev.h b/include/linux/backing-dev.h
index bd0e3c6..90e677a 100644
--- a/include/linux/backing-dev.h
+++ b/include/linux/backing-dev.h
@@ -103,7 +103,7 @@ int bdi_register_dev(struct backing_dev_info *bdi, dev_t dev);
void bdi_unregister(struct backing_dev_info *bdi);
int bdi_setup_and_register(struct backing_dev_info *, char *, unsigned int);
void bdi_start_writeback(struct backing_dev_info *bdi, struct super_block *sb,
- long nr_pages);
+ long nr_pages, int sb_locked);
int bdi_writeback_task(struct bdi_writeback *wb);
int bdi_has_dirty_io(struct backing_dev_info *bdi);
diff --git a/include/linux/writeback.h b/include/linux/writeback.h
index 36520de..3cd39b0 100644
--- a/include/linux/writeback.h
+++ b/include/linux/writeback.h
@@ -19,6 +19,7 @@ extern struct list_head inode_unused;
enum writeback_sync_modes {
WB_SYNC_NONE, /* Don't wait on anything */
WB_SYNC_ALL, /* Wait on every mapping */
+ WB_SYNC_NONE_PIN, /* Like WB_SYNC_NONE, but s_umount held */
};
/*
@@ -73,6 +74,7 @@ struct writeback_control {
struct bdi_writeback;
int inode_wait(void *);
void writeback_inodes_sb(struct super_block *);
+void writeback_inodes_sb_locked(struct super_block *);
int writeback_inodes_sb_if_idle(struct super_block *);
void sync_inodes_sb(struct super_block *);
void writeback_inodes_wbc(struct writeback_control *wbc);
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 0b19943..49d3508 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -597,7 +597,7 @@ static void balance_dirty_pages(struct address_space *mapping,
(!laptop_mode && ((global_page_state(NR_FILE_DIRTY)
+ global_page_state(NR_UNSTABLE_NFS))
> background_thresh)))
- bdi_start_writeback(bdi, NULL, 0);
+ bdi_start_writeback(bdi, NULL, 0, 0);
}
void set_page_dirty_balance(struct page *page, int page_mkwrite)
--
Jens Axboe
--
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