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    SubjectRe: [PATCH] vfs: Merge sync_supers(), sync_filesystems() and sync_blockdevs()
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    On Wed, 2009-04-22 at 17:56 +0200, Jan Kara wrote:
    > These three functions are quite similar so merge them to save superblock list
    > traversal code. As a bonus we get livelock avoidance for all these superblock
    > traversals. Also remove the condition that if wait == 0 and sb->s_dirt is
    > not set, then ->sync_fs() is not called. This does not really make much sence
    > since s_dirt is generally used by filesystem to mean that ->write_super() needs
    > to be called. But ->sync_fs() does different things. I even suspect that some
    > filesystems (btrfs?) sets s_dirt just to fool this logic.
    >
    > Signed-off-by: Jan Kara <jack@suse.cz>
    > ---
    > fs/super.c | 104 ++++++++++++++------------------------------------
    > fs/sync.c | 12 ++++--
    > include/linux/fs.h | 11 +++--
    > mm/page-writeback.c | 2 +-
    > 4 files changed, 44 insertions(+), 85 deletions(-)
    >
    > diff --git a/fs/super.c b/fs/super.c
    > index 4f56333..6d6209e 100644
    > --- a/fs/super.c
    > +++ b/fs/super.c
    > @@ -413,58 +413,24 @@ void drop_super(struct super_block *sb)
    >
    > EXPORT_SYMBOL(drop_super);
    >
    > -static inline void write_super(struct super_block *sb)
    > -{
    > - lock_super(sb);
    > - if (sb->s_root && sb->s_dirt)
    > - if (sb->s_op->write_super)
    > - sb->s_op->write_super(sb);
    > - unlock_super(sb);
    > -}
    > -
    > -/*
    > - * Note: check the dirty flag before waiting, so we don't
    > - * hold up the sync while mounting a device. (The newly
    > - * mounted device won't need syncing.)
    > - */
    > -void sync_supers(void)
    > -{
    > - struct super_block *sb;
    > -
    > - spin_lock(&sb_lock);
    > -restart:
    > - list_for_each_entry(sb, &super_blocks, s_list) {
    > - if (sb->s_dirt) {
    > - sb->s_count++;
    > - spin_unlock(&sb_lock);
    > - down_read(&sb->s_umount);
    > - write_super(sb);
    > - up_read(&sb->s_umount);
    > - spin_lock(&sb_lock);
    > - if (__put_super_and_need_restart(sb))
    > - goto restart;
    > - }
    > - }
    > - spin_unlock(&sb_lock);
    > -}
    > -
    > /*
    > - * Call the ->sync_fs super_op against all filesystems which are r/w and
    > - * which implement it.
    > + * Call the ->write_super, ->sync_fs, sync_blockdev() against all filesystems
    > + * which are r/w and which implement it.
    > *
    > - * 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
    > - * is used only here. We set it against all filesystems and then clear it as
    > - * we sync them. So redirtied filesystems are skipped.
    > + * This operation is careful to avoid the livelock. Currently it's not so easy
    > + * to trigger as it used to be - we restart scanning the superblock list only
    > + * if the superblock got unmounted under us - but it's better to be safe and it
    > + * doesn't cost us much. 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
    > - * flags again, which will cause process A to resync everything. Fix that with
    > - * a local mutex.
    > + * 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)
    > +void sync_filesystems(int what, int wait)
    > {
    > struct super_block *sb;
    > static DEFINE_MUTEX(mutex);
    > @@ -472,26 +438,37 @@ 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)
    > + if ((!sb->s_op->sync_fs || !(what & FSSYNC_FS)) &&
    > + (!sb->s_op->write_super || !(what & FSSYNC_SUPER)) &&
    > + !(what & FSSYNC_BDEV))

    Ditto. There should be a check for sb->s_bdev here too...

    > 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++;
    > spin_unlock(&sb_lock);
    > down_read(&sb->s_umount);
    > async_synchronize_full_domain(&sb->s_async_list);
    > - if (sb->s_root && (wait || sb->s_dirt))
    > - sb->s_op->sync_fs(sb, wait);
    > + if (sb->s_root) {
    > + lock_super(sb);
    > + if (what & FSSYNC_SUPER && sb->s_dirt &&
    > + sb->s_op->write_super)
    > + sb->s_op->write_super(sb);
    > + unlock_super(sb);
    > + if (what & FSSYNC_FS && sb->s_op->sync_fs)
    > + sb->s_op->sync_fs(sb, wait);
    > + if (what & FSSYNC_BDEV)
    > + sync_blockdev(sb->s_bdev);
    > + }
    > up_read(&sb->s_umount);
    > /* restart only when sb is no longer on the list */
    > spin_lock(&sb_lock);
    > @@ -502,29 +479,6 @@ 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) {
    > - 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..3c677ee 100644
    > --- a/fs/sync.c
    > +++ b/fs/sync.c
    > @@ -26,11 +26,15 @@ static void do_sync(unsigned long wait)
    > wakeup_pdflush(0);
    > sync_inodes(0); /* All mappings, inodes and their blockdevs */
    > vfs_dq_sync(NULL);
    > + sync_filesystems(FSSYNC_FS, 0); /* Start syncing the filesystems */
    > + /*
    > + * We have to reliably submit IO for all the inodes before writing
    > + * super blocks and calling sync_fs(). Otherwise superblock could miss
    > + * some updates or journal could still have uncommitted data.
    > + */
    > 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();
    > + /* Waitingly sync the filesystems */
    > + sync_filesystems(FSSYNC_SUPER | FSSYNC_FS | FSSYNC_BDEV, wait);
    > if (!wait)
    > printk("Emergency Sync complete\n");
    > if (unlikely(laptop_mode))
    > diff --git a/include/linux/fs.h b/include/linux/fs.h
    > index 4bad02e..2758e75 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 +1942,6 @@ 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 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);
    > @@ -1952,7 +1951,6 @@ extern int fsync_no_super(struct block_device *);
    > #else
    > static inline void bd_forget(struct inode *inode) {}
    > static inline int sync_blockdev(struct block_device *bdev) { return 0; }
    > -static inline void sync_blockdevs(void) { }
    > static inline void invalidate_bdev(struct block_device *bdev) {}
    >
    > static inline struct super_block *freeze_bdev(struct block_device *sb)
    > @@ -2082,8 +2080,11 @@ extern int filemap_fdatawrite_range(struct address_space *mapping,
    > loff_t start, loff_t end);
    >
    > extern int vfs_fsync(struct file *file, struct dentry *dentry, int datasync);
    > -extern void sync_supers(void);
    > -extern void sync_filesystems(int wait);
    > +/* Flags telling what should be synced by sync_filesystems() */
    > +#define FSSYNC_FS 1
    > +#define FSSYNC_SUPER 2
    > +#define FSSYNC_BDEV 4
    > +extern void sync_filesystems(int what, int wait);
    > extern void __fsync_super(struct super_block *sb);
    > extern void emergency_sync(void);
    > extern void emergency_remount(void);
    > diff --git a/mm/page-writeback.c b/mm/page-writeback.c
    > index 30351f0..05a2948 100644
    > --- a/mm/page-writeback.c
    > +++ b/mm/page-writeback.c
    > @@ -768,7 +768,7 @@ static void wb_kupdate(unsigned long arg)
    > .range_cyclic = 1,
    > };
    >
    > - sync_supers();
    > + sync_filesystems(FSSYNC_SUPER, 0);
    >
    > oldest_jif = jiffies - msecs_to_jiffies(dirty_expire_interval);
    > start_jif = jiffies;



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