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Subject[PATCH 4/4] vfs: Make sys_sync() use fsync_super() (version 3)
Date
It is unnecessarily fragile to have two places (fsync_super() and do_sync())
doing data integrity sync of the filesystem. Alter __fsync_super() to
accommodate needs of both callers and use it. So after this patch
__fsync_super() is the only place where we gather all the calls needed to
properly send all data on a filesystem to disk.

Nice bonus is that we get a complete livelock avoidance and write_supers()
is now only used for periodic writeback of superblocks.

Signed-off-by: Jan Kara <jack@suse.cz>
---
fs/block_dev.c | 15 ++++++---
fs/fs-writeback.c | 49 -------------------------------
fs/super.c | 70 +++++++++++++++------------------------------
fs/sync.c | 31 ++++++-------------
include/linux/fs.h | 4 +-
include/linux/writeback.h | 1 -
6 files changed, 45 insertions(+), 125 deletions(-)
diff --git a/fs/block_dev.c b/fs/block_dev.c
index 48d1290..2609cce 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -175,17 +175,22 @@ blkdev_direct_IO(int rw, struct kiocb *iocb, const struct iovec *iov,
iov, offset, nr_segs, blkdev_get_blocks, NULL);
}

+int __sync_blockdev(struct block_device *bdev, int wait)
+{
+ if (!bdev)
+ return 0;
+ if (!wait)
+ return filemap_flush(bdev->bd_inode->i_mapping);
+ return filemap_write_and_wait(bdev->bd_inode->i_mapping);
+}
+
/*
* Write out and wait upon all the dirty data associated with a block
* device via its mapping. Does not take the superblock lock.
*/
int sync_blockdev(struct block_device *bdev)
{
- int ret = 0;
-
- if (bdev)
- ret = filemap_write_and_wait(bdev->bd_inode->i_mapping);
- return ret;
+ return __sync_blockdev(bdev, 1);
}
EXPORT_SYMBOL(sync_blockdev);

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 91013ff..e0fb2e7 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -679,55 +679,6 @@ void sync_inodes_sb(struct super_block *sb, int wait)
}

/**
- * sync_inodes - writes all inodes to disk
- * @wait: wait for completion
- *
- * sync_inodes() goes through each super block's dirty inode list, writes the
- * inodes out, waits on the writeout and puts the inodes back on the normal
- * list.
- *
- * This is for sys_sync(). fsync_dev() uses the same algorithm. The subtle
- * part of the sync functions is that the blockdev "superblock" is processed
- * last. This is because the write_inode() function of a typical fs will
- * perform no I/O, but will mark buffers in the blockdev mapping as dirty.
- * What we want to do is to perform all that dirtying first, and then write
- * back all those inode blocks via the blockdev mapping in one sweep. So the
- * additional (somewhat redundant) sync_blockdev() calls here are to make
- * sure that really happens. Because if we call sync_inodes_sb(wait=1) with
- * outstanding dirty inodes, the writeback goes block-at-a-time within the
- * filesystem's write_inode(). This is extremely slow.
- */
-static void __sync_inodes(int wait)
-{
- struct super_block *sb;
-
- spin_lock(&sb_lock);
-restart:
- list_for_each_entry(sb, &super_blocks, s_list) {
- sb->s_count++;
- spin_unlock(&sb_lock);
- down_read(&sb->s_umount);
- if (sb->s_root) {
- sync_inodes_sb(sb, wait);
- sync_blockdev(sb->s_bdev);
- }
- up_read(&sb->s_umount);
- spin_lock(&sb_lock);
- if (__put_super_and_need_restart(sb))
- goto restart;
- }
- spin_unlock(&sb_lock);
-}
-
-void sync_inodes(int wait)
-{
- __sync_inodes(0);
-
- if (wait)
- __sync_inodes(1);
-}
-
-/**
* write_inode_now - write an inode to disk
* @inode: inode to write to disk
* @sync: whether the write should be synchronous or not
diff --git a/fs/super.c b/fs/super.c
index 4c92068..b5d7dfb 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -258,23 +258,23 @@ EXPORT_SYMBOL(lock_super);
EXPORT_SYMBOL(unlock_super);

/*
- * Write out and wait upon all dirty data associated with this
- * superblock. Filesystem data as well as the underlying block
- * device. Takes the superblock lock. Requires a second blkdev
- * flush by the caller to complete the operation.
+ * Do the filesystem syncing work. For simple filesystems sync_inodes_sb(sb, 0)
+ * just dirties buffers with inodes so we have to submit IO for these buffers
+ * via __sync_blockdev(). This also speeds up the wait == 1 case since in that
+ * case write_inode() functions do sync_dirty_buffer() and thus effectively
+ * write one block at a time.
*/
-static int __fsync_super(struct super_block *sb)
+static int __fsync_super(struct super_block *sb, int wait)
{
- sync_inodes_sb(sb, 0);
vfs_dq_sync(sb);
- sync_inodes_sb(sb, 1);
+ sync_inodes_sb(sb, wait);
lock_super(sb);
if (sb->s_dirt && sb->s_op->write_super)
sb->s_op->write_super(sb);
unlock_super(sb);
if (sb->s_op->sync_fs)
- sb->s_op->sync_fs(sb, 1);
- sync_blockdev(sb->s_bdev);
+ sb->s_op->sync_fs(sb, wait);
+ return __sync_blockdev(sb->s_bdev, wait);
}

/*
@@ -284,7 +284,12 @@ static int __fsync_super(struct super_block *sb)
*/
int fsync_super(struct super_block *sb)
{
- return __fsync_super(sb);
+ int ret;
+
+ ret = __fsync_super(sb, 0);
+ if (ret < 0)
+ return ret;
+ return __fsync_super(sb, 1);
}
EXPORT_SYMBOL_GPL(fsync_super);

@@ -448,20 +453,18 @@ restart:
}

/*
- * Call the ->sync_fs super_op against all filesystems which are r/w and
- * which implement it.
+ * Sync all the data for all the filesystems (called by sys_sync() and
+ * emergency sync)
*
* This operation is careful to avoid the livelock which could easily happen
- * if two or more filesystems are being continuously dirtied. s_need_sync_fs
+ * if two or more filesystems are being continuously dirtied. s_need_sync
* is used only here. We set it against all filesystems and then clear it as
* we sync them. So redirtied filesystems are skipped.
*
* But if process A is currently running sync_filesystems and then process B
- * calls sync_filesystems as well, process B will set all the s_need_sync_fs
+ * calls sync_filesystems as well, process B will set all the s_need_sync
* flags again, which will cause process A to resync everything. Fix that with
* a local mutex.
- *
- * (Fabian) Avoid sync_fs with clean fs & wait mode 0
*/
void sync_filesystems(int wait)
{
@@ -471,18 +474,16 @@ void sync_filesystems(int wait)
mutex_lock(&mutex); /* Could be down_interruptible */
spin_lock(&sb_lock);
list_for_each_entry(sb, &super_blocks, s_list) {
- if (!sb->s_op->sync_fs)
- continue;
if (sb->s_flags & MS_RDONLY)
continue;
- sb->s_need_sync_fs = 1;
+ sb->s_need_sync = 1;
}

restart:
list_for_each_entry(sb, &super_blocks, s_list) {
- if (!sb->s_need_sync_fs)
+ if (!sb->s_need_sync)
continue;
- sb->s_need_sync_fs = 0;
+ sb->s_need_sync = 0;
if (sb->s_flags & MS_RDONLY)
continue; /* hm. Was remounted r/o meanwhile */
sb->s_count++;
@@ -490,7 +491,7 @@ restart:
down_read(&sb->s_umount);
async_synchronize_full_domain(&sb->s_async_list);
if (sb->s_root)
- sb->s_op->sync_fs(sb, wait);
+ __fsync_super(sb, wait);
up_read(&sb->s_umount);
/* restart only when sb is no longer on the list */
spin_lock(&sb_lock);
@@ -501,31 +502,6 @@ restart:
mutex_unlock(&mutex);
}

-/*
- * Sync all block devices underlying some superblock
- */
-void sync_blockdevs(void)
-{
- struct super_block *sb;
-
- spin_lock(&sb_lock);
-restart:
- list_for_each_entry(sb, &super_blocks, s_list) {
- if (!sb->s_bdev)
- continue;
- sb->s_count++;
- spin_unlock(&sb_lock);
- down_read(&sb->s_umount);
- if (sb->s_root)
- sync_blockdev(sb->s_bdev);
- up_read(&sb->s_umount);
- spin_lock(&sb_lock);
- if (__put_super_and_need_restart(sb))
- goto restart;
- }
- spin_unlock(&sb_lock);
-}
-
/**
* get_super - get the superblock of a device
* @bdev: device to get the superblock for
diff --git a/fs/sync.c b/fs/sync.c
index fa14e42..86c6a86 100644
--- a/fs/sync.c
+++ b/fs/sync.c
@@ -17,35 +17,24 @@
#define VALID_FLAGS (SYNC_FILE_RANGE_WAIT_BEFORE|SYNC_FILE_RANGE_WRITE| \
SYNC_FILE_RANGE_WAIT_AFTER)

-/*
- * sync everything. Start out by waking pdflush, because that writes back
- * all queues in parallel.
- */
-static void do_sync(unsigned long wait)
+SYSCALL_DEFINE0(sync)
{
- wakeup_pdflush(0);
- sync_inodes(0); /* All mappings, inodes and their blockdevs */
- vfs_dq_sync(NULL);
- sync_inodes(wait); /* Mappings, inodes and blockdevs, again. */
- sync_supers(); /* Write the superblocks */
- sync_filesystems(0); /* Start syncing the filesystems */
- sync_filesystems(wait); /* Waitingly sync the filesystems */
- sync_blockdevs();
- if (!wait)
- printk("Emergency Sync complete\n");
+ sync_filesystems(0);
+ sync_filesystems(1);
if (unlikely(laptop_mode))
laptop_sync_completion();
-}
-
-SYSCALL_DEFINE0(sync)
-{
- do_sync(1);
return 0;
}

static void do_sync_work(struct work_struct *work)
{
- do_sync(0);
+ /*
+ * Sync twice to reduce the possibility we skipped some inodes / pages
+ * because they were temporarily locked
+ */
+ sync_filesystems(0);
+ sync_filesystems(0);
+ printk("Emergency Sync complete\n");
kfree(work);
}

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 47a67c9..be2be8d 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1321,7 +1321,7 @@ struct super_block {
struct rw_semaphore s_umount;
struct mutex s_lock;
int s_count;
- int s_need_sync_fs;
+ int s_need_sync;
atomic_t s_active;
#ifdef CONFIG_SECURITY
void *s_security;
@@ -1942,7 +1942,7 @@ extern void bdput(struct block_device *);
extern struct block_device *open_by_devnum(dev_t, fmode_t);
extern void invalidate_bdev(struct block_device *);
extern int sync_blockdev(struct block_device *bdev);
-extern void sync_blockdevs(void);
+extern int __sync_blockdev(struct block_device *bdev, int wait);
extern struct super_block *freeze_bdev(struct block_device *);
extern void emergency_thaw_all(void);
extern int thaw_bdev(struct block_device *bdev, struct super_block *sb);
diff --git a/include/linux/writeback.h b/include/linux/writeback.h
index 9c1ed1f..943d1c9 100644
--- a/include/linux/writeback.h
+++ b/include/linux/writeback.h
@@ -79,7 +79,6 @@ struct writeback_control {
void writeback_inodes(struct writeback_control *wbc);
int inode_wait(void *);
void sync_inodes_sb(struct super_block *, int wait);
-void sync_inodes(int wait);

/* writeback.h requires fs.h; it, too, is not included from here. */
static inline void wait_on_inode(struct inode *inode)
--
1.6.0.2


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