On Wed, Jan 10, 2018 at 2:32 PM, 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. By doing this, It is able to reduce cost of synchronization
> and will not seriously impact IO performance of irrelative processes.
>
> Signed-off-by: Chengguang Xu <cgxu519@xxxxxxxxxx>
> ---
> Changes since v1:
> - If upper filesystem is readonly mode then skip synchronization.
> - Introduce global wait list to replace temporary wait list for
> concurrent synchronization.
>
Looks ok. A few more suggestions below.
> fs/overlayfs/ovl_entry.h | 5 +++
> fs/overlayfs/super.c | 105 +++++++++++++++++++++++++++++++++++++++++++++--
> 2 files changed, 107 insertions(+), 3 deletions(-)
>
> diff --git a/fs/overlayfs/ovl_entry.h b/fs/overlayfs/ovl_entry.h
> index 9d0bc03..ff935da 100644
> --- a/fs/overlayfs/ovl_entry.h
> +++ b/fs/overlayfs/ovl_entry.h
> @@ -52,6 +52,9 @@ struct ovl_fs {
> /* Did we take the inuse lock? */
> bool upperdir_locked;
> bool workdir_locked;
> + /* ovl inode sync list and lock */
> + spinlock_t ovl_sync_list_lock;
> + struct list_head ovl_sync_list;
> };
>
> /* private information held for every overlayfs dentry */
> @@ -80,6 +83,8 @@ 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..c7b788b 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 +255,97 @@ 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.
> + */
> +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;
> +
> + struct writeback_control wbc = {
> + .sync_mode = WB_SYNC_ALL,
> + .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);
> + spin_lock(&ofs->ovl_sync_list_lock);
> + if (list_empty(&OVL_I(inode)->sync_list))
> + list_add(&OVL_I(inode)->sync_list, &ofs->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);
> +
You may want to consider factoring out helpers ovl_sync_inodes()
and maybe also ovl_writeback_inodes() ovl_wait_inodes(),
so the code in nicer and resembles generic helper structure.
> + mutex_lock(&upper_sb->s_sync_lock);
So it makes some sense to use upper_sb sync lock
to synchronize with callers of sync(2)/syncfs(2) on the upper fs,
but that could result syncfs() on overlay to take much longer if
there is a syncfs() on upper fs in progress.
If you think of ovl_sync_filesystem() as a selective fsync() iterator,
then it might make more sense to synchronize overlayfs syncfs()
callers with overlay sb sync lock.
Thanks,
Amir.
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