On Mon, Jan 8, 2018 at 10:41 AM, Chengguang Xu <cgxu519@xxxxxxxxxx> wrote:
> Currently syncfs(2) call on overlayfs just simply call sync_filesystem()
> on upper_sb to synchronize whole dirty inodes in upper filesystem
> regardless of the overlay ownership of the inode. It has obvious
> shortcomings as below.
>
> (1) Performance
> Synchronization is probably heavy in most cases, especially when upper
> filesystem is not dedicated to target overlayfs.
>
> (2) Interference
> Unplanned synchronization will probably impact IO performance of the
> processes which use other overlayfs on same upper filesystem or directly
> use upper filesystem.
>
> This patch iterates overlay inodes to only sync target dirty inodes in
> upper filesystem and wait on temporary waiting list to avoid waiting on
> inodes that have started writeback afterwards. By doing this,
> it is able to reduce cost of synchronization and will not seriously impact
> IO performance of irrelative processes.
This looks like a very good improvement!
>
> Signed-off-by: Chengguang Xu <cgxu519@xxxxxxxxxx>
> ---
> fs/overlayfs/ovl_entry.h | 4 ++
> fs/overlayfs/super.c | 101 +++++++++++++++++++++++++++++++++++++++++++++--
> 2 files changed, 102 insertions(+), 3 deletions(-)
>
> diff --git a/fs/overlayfs/ovl_entry.h b/fs/overlayfs/ovl_entry.h
> index 9d0bc03..e92c425 100644
> --- a/fs/overlayfs/ovl_entry.h
> +++ b/fs/overlayfs/ovl_entry.h
> @@ -52,6 +52,8 @@ struct ovl_fs {
> /* Did we take the inuse lock? */
> bool upperdir_locked;
> bool workdir_locked;
> + /* ovl inode sync list lock */
> + spinlock_t ovl_sync_list_lock;
> };
>
> /* private information held for every overlayfs dentry */
> @@ -80,6 +82,8 @@ struct ovl_inode {
>
> /* synchronize copy up and more */
> struct mutex lock;
> + /* ovl inode sync list */
> + struct list_head sync_list;
> };
>
> static inline struct ovl_inode *OVL_I(struct inode *inode)
> diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
> index 76440fe..17f1406 100644
> --- a/fs/overlayfs/super.c
> +++ b/fs/overlayfs/super.c
> @@ -17,6 +17,8 @@
> #include <linux/statfs.h>
> #include <linux/seq_file.h>
> #include <linux/posix_acl_xattr.h>
> +#include <linux/writeback.h>
> +#include <linux/blkdev.h>
> #include "overlayfs.h"
>
> MODULE_AUTHOR("Miklos Szeredi <miklos@xxxxxxxxxx>");
> @@ -195,6 +197,7 @@ static struct inode *ovl_alloc_inode(struct super_block *sb)
> oi->__upperdentry = NULL;
> oi->lower = NULL;
> mutex_init(&oi->lock);
> + INIT_LIST_HEAD(&oi->sync_list);
>
> return &oi->vfs_inode;
> }
> @@ -252,6 +255,96 @@ static void ovl_put_super(struct super_block *sb)
> ovl_free_fs(ofs);
> }
>
> +/**
> + * ovl_sync_filesystem
> + * @sb: The overlayfs super block
> + *
> + * Sync underlying dirty inodes in upper filesystem and wait for completion.
> + * Use temporary wait list to avoid waiting on inodes that have started
> + * writeback after this point.
> + */
> +static int ovl_sync_filesystem(struct super_block *sb)
> +{
> + struct ovl_fs *ofs = sb->s_fs_info;
> + struct super_block *upper_sb = ofs->upper_mnt->mnt_sb;
> + struct ovl_inode *oi, *oi_next;
> + struct inode *inode, *i_next;
> + struct inode *upper_inode;
> + struct blk_plug plug;
> + LIST_HEAD(ovl_sync_list);
> +
> + struct writeback_control wbc = {
> + .sync_mode = WB_SYNC_ALL,
Shouldn't this be WB_SYNC_NONE? We are explicitly waiting
inodes on the list afterwards...
> + .for_sync = 1,
> + .range_start = 0,
> + .range_end = LLONG_MAX,
> + .nr_to_write = LONG_MAX,
> + };
> +
> + blk_start_plug(&plug);
> + spin_lock(&sb->s_inode_list_lock);
> + list_for_each_entry_safe(inode, i_next, &sb->s_inodes, i_sb_list) {
> + upper_inode = ovl_inode_upper(inode);
> + if (!upper_inode)
> + continue;
> +
> + spin_lock(&upper_inode->i_lock);
> + if (upper_inode->i_state & (I_NEW | I_FREEING | I_WILL_FREE) ||
> + list_empty(&upper_inode->i_io_list)) {
> + spin_unlock(&upper_inode->i_lock);
> + continue;
> + }
> + spin_unlock(&upper_inode->i_lock);
> +
> + if (!igrab(inode))
> + continue;
> +
> + if (!igrab(upper_inode)) {
> + iput(inode);
> + continue;
> + }
> + spin_unlock(&sb->s_inode_list_lock);
> +
> + sync_inode(upper_inode, &wbc);
Maybe allowed to call sync_inode() under s_inode_list_lock
if using WB_SYNC_NONE???
Do we not need to check return value from sync_inode()?
I bet it could be EROFS if upper fs was remounted readonly
and we did not check for that before starting to sync...
> + spin_lock(&ofs->ovl_sync_list_lock);
> + if (list_empty(&OVL_I(inode)->sync_list))
> + list_add(&OVL_I(inode)->sync_list, &ovl_sync_list);
> + else {
> + iput(upper_inode);
> + iput(inode);
> + }
> + spin_unlock(&ofs->ovl_sync_list_lock);
> +
> + if (need_resched()) {
> + blk_finish_plug(&plug);
> + cond_resched();
> + blk_start_plug(&plug);
> + }
> + spin_lock(&sb->s_inode_list_lock);
> + }
> + spin_unlock(&sb->s_inode_list_lock);
> + blk_finish_plug(&plug);
> +
> + mutex_lock(&upper_sb->s_sync_lock);
I suppose mutex_lock_interruptible() is in order.
> + list_for_each_entry_safe(oi, oi_next, &ovl_sync_list, sync_list) {
> + upper_inode = ovl_inode_upper(&oi->vfs_inode);
> +
> + filemap_fdatawait_keep_errors(upper_inode->i_mapping);
> + list_del_init(&oi->sync_list);
> + iput(upper_inode);
> + iput(&oi->vfs_inode);
> +
> + if (need_resched())
> + cond_resched();
> + }
> + mutex_unlock(&upper_sb->s_sync_lock);
> +
> + if (upper_sb->s_op->sync_fs)
> + upper_sb->s_op->sync_fs(upper_sb, 1);
> +
> + return sync_blockdev(upper_sb->s_bdev);
> +}
> +
> /* Sync real dirty inodes in upper filesystem (if it exists) */
> static int ovl_sync_fs(struct super_block *sb, int wait)
> {
> @@ -266,8 +359,8 @@ static int ovl_sync_fs(struct super_block *sb, int wait)
> * If this is a sync(2) call or an emergency sync, all the super blocks
> * will be iterated, including upper_sb, so no need to do anything.
> *
> - * If this is a syncfs(2) call, then we do need to call
> - * sync_filesystem() on upper_sb, but enough if we do it when being
> + * If this is a syncfs(2) call, then we do need to call sync_inode()
> + * on underlying dirty upper_inode, but enough if we do it when being
> * called with wait == 1.
> */
> if (!wait)
> @@ -276,7 +369,7 @@ static int ovl_sync_fs(struct super_block *sb, int wait)
> upper_sb = ofs->upper_mnt->mnt_sb;
>
> down_read(&upper_sb->s_umount);
> - ret = sync_filesystem(upper_sb);
> + ret = ovl_sync_filesystem(sb);
Note that with this change, we are skipping the test sb_rdonly(upper_sb)
(inside sync_filesystem()). We should probably skip syncing in that case.
Thanks,
Amir.
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