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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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