On Mon, Jan 15, 2018 at 9:33 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. In the use case of
> container, when multiple containers using the same underlying upper
> filesystem, it has some shortcomings as below.
>
> (1) Performance
> Synchronization is probably heavy because it actually syncs unnecessary
> inodes for target overlayfs.
>
> (2) Interference
> Unplanned synchronization will probably impact IO performance of
> irrelative container processes on the other overlayfs.
s/irrelative/unrelated
>
> This patch iterates upper inode list in overlayfs to only sync target
> dirty inodes and wait for completion. By doing this, It is able to reduce
> cost of synchronization and will not seriously impact IO performance of
> irrelative processes. In special case, when having very few dirty inodes
unrelated processes
> and a lot of clean upper inodes in overlayfs, then iteration may waste
> time so that synchronization is slower than before, but we think the
> potential for performance improvement far surpass the potential for
> performance regression in most cases.
>
> Signed-off-by: Chengguang Xu <cgxu519@xxxxxxxxxx>
Beyond Miklos's comment on removing inodes from list, a few more
comments and nits below.
> ---
> Changes since v3:
> - Introduce upper_inode list to organize ovl indoe which has upper inode.
> - Avoid iput operation inside spin lock.
> - Change list iteration method for safety.
> - Reuse inode->i_wb_list and sb->s_inodes_wb to save memory.
> - Modify commit log and add more comments to the code.
>
> Changes since v2:
> - Decoupling with syncfs of upper fs by taking sb->s_sync_lock of
> overlayfs.
> - Factoring out sync operation to different helpers for easy understanding.
>
> Changes since v1:
> - If upper filesystem is readonly mode then skip synchronization.
> - Introduce global wait list to replace temporary wait list for
> concurrent synchronization.
>
> fs/overlayfs/ovl_entry.h | 5 ++
> fs/overlayfs/super.c | 167 ++++++++++++++++++++++++++++++++++++++++++++++-
> fs/overlayfs/util.c | 13 +++-
> 3 files changed, 181 insertions(+), 4 deletions(-)
>
> diff --git a/fs/overlayfs/ovl_entry.h b/fs/overlayfs/ovl_entry.h
> index 9d0bc03..53909ba 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;
> + /* upper inode list and lock */
> + spinlock_t upper_inodes_lock;
> + struct list_head upper_inodes;
> };
>
> /* private information held for every overlayfs dentry */
> @@ -80,6 +83,8 @@ struct ovl_inode {
>
> /* synchronize copy up and more */
> struct mutex lock;
> + /* upper inodes list */
> + struct list_head upper_inodes_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..39fb331 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->upper_inodes_list);
>
> return &oi->vfs_inode;
> }
> @@ -252,6 +255,159 @@ static void ovl_put_super(struct super_block *sb)
> ovl_free_fs(ofs);
> }
>
> +/**
> + * ovl_sync_inodes
> + * @sb: The overlayfs super block
> + *
> + * upper_inodes list is used for organizing ovl inode which has upper inode,
> + * by iterating the list to looking for and syncing dirty upper inode.
> + *
> + * When starting syncing inode, we add the inode to wait list explicitly,
> + * in order to save memory we reuse inode->i_wb_list and sb->s_inodes_wb,
> + * so those fields have slightly differnt meanings in overlayfs.
> + */
> +static void ovl_sync_inodes(struct super_block *sb)
> +{
> + struct ovl_fs *ofs = sb->s_fs_info;
> + struct ovl_inode *oi;
> + struct inode *inode, *iput_inode = NULL;
> + 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(&ofs->upper_inodes_lock);
> + list_for_each_entry(oi, &ofs->upper_inodes, upper_inodes_list) {
> + inode = &oi->vfs_inode;
> +
> + spin_lock(&inode->i_lock);
> + if (inode->i_state & (I_NEW | I_FREEING | I_WILL_FREE)) {
> + spin_unlock(&inode->i_lock);
> + continue;
> + }
> +
> + if (!atomic_read(&inode->i_count)) {
> + spin_unlock(&inode->i_lock);
> + continue;
> + }
> +
> + ihold(inode);
> + upper_inode = ovl_inode_upper(inode);
> + spin_unlock(&inode->i_lock);
> +
> + spin_lock(&upper_inode->i_lock);
> + if (!(upper_inode->i_state & I_DIRTY_ALL)) {
> + iput_inode = inode;
> + spin_unlock(&upper_inode->i_lock);
> + continue;
continue may not get to do anything with iput_inode before
assigning the next non dirty inode to iput_inode.
Didn't you mean to call iput(iput_inode) sooner?
> + }
> + spin_unlock(&upper_inode->i_lock);
> + spin_unlock(&ofs->upper_inodes_lock);
> +
> + if (iput_inode)
> + iput(iput_inode);
> +
> + sync_inode(upper_inode, &wbc);
> +
> + spin_lock(&sb->s_inode_wblist_lock);
> + if (list_empty(&inode->i_wb_list)) {
> + list_add_tail(&inode->i_wb_list, &sb->s_inodes_wb);
> + /*
> + * Hold referece of ovl inode
> + * until finishing waiting on the inode.
> + */
Wrong indentation and I think you should move this comment outside
if case and drop the {}, in any case if you have {} in if you should also
have {} in else.
> + iput_inode = NULL;
> + } else
> + iput_inode = inode;
> + spin_unlock(&sb->s_inode_wblist_lock);
> +
> + if (need_resched()) {
> + blk_finish_plug(&plug);
> + cond_resched();
> + blk_start_plug(&plug);
> + }
> +
> + spin_lock(&ofs->upper_inodes_lock);
> + }
> + spin_unlock(&ofs->upper_inodes_lock);
> + blk_finish_plug(&plug);
> +
> + if (iput_inode)
> + iput(iput_inode);
> +}
> +
> +static void ovl_wait_inodes(struct super_block *sb)
> +{
> + LIST_HEAD(sync_list);
> + struct inode *inode;
> + struct inode *upper_inode;
> +
> + mutex_lock(&sb->s_sync_lock);
> + /*
> + * Splice the sync wait list onto a temporary list to avoid waiting on
> + * inodes that have started writeback after this point.
> + */
> + spin_lock(&sb->s_inode_wblist_lock);
> + list_splice_init(&sb->s_inodes_wb, &sync_list);
> + spin_unlock(&sb->s_inode_wblist_lock);
> +
> + while (!list_empty(&sync_list)) {
> + inode = list_first_entry(&sync_list, struct inode, i_wb_list);
> + list_del_init(&inode->i_wb_list);
> +
> + spin_lock(&inode->i_lock);
> + if (inode->i_state & (I_NEW | I_FREEING | I_WILL_FREE)) {
> + spin_unlock(&inode->i_lock);
> + iput(inode);
> + continue;
> + }
> +
> + upper_inode = ovl_inode_upper(inode);
> + spin_unlock(&inode->i_lock);
> +
> + if (!mapping_tagged(upper_inode->i_mapping,
> + PAGECACHE_TAG_WRITEBACK)) {
> + iput(inode);
> + continue;
> + }
> +
> + filemap_fdatawait_keep_errors(upper_inode->i_mapping);
> +
> + if (need_resched())
> + cond_resched();
> +
> + iput(inode);
> + }
> + mutex_unlock(&sb->s_sync_lock);
> +}
> +
> +/**
> + * 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;
> +
> + ovl_sync_inodes(sb);
> + ovl_wait_inodes(sb);
> +
> + 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)
> {
> @@ -266,17 +422,19 @@ 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)
> return 0;
>
> upper_sb = ofs->upper_mnt->mnt_sb;
> + if (sb_rdonly(upper_sb))
> + return 0;
>
> down_read(&upper_sb->s_umount);
> - ret = sync_filesystem(upper_sb);
> + ret = ovl_sync_filesystem(sb);
> up_read(&upper_sb->s_umount);
The boundaries between ovl_sync_fs() and ovl_sync_filesystem() are now very
strange.. taking upper_sb s_umount lock outside and passing in sb etc.
I am not sure that ovl_sync/wait_inodes() should be called under upper_sb
s_umount lock at all. Remember we are already holding sb s_umount lock.
I suggest, now that ovl_sync/wait_inodes() have been factored out, to squash
ovl_sync_filesystem() back into ovl_sync_fs() and hold the s_umount lock
only around s_op->sync_fs() and sync_blockdev().
>
> return ret;
> @@ -1202,6 +1360,9 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
> if (!ofs)
> goto out;
>
> + spin_lock_init(&ofs->upper_inodes_lock);
> + INIT_LIST_HEAD(&ofs->upper_inodes);
> +
> ofs->creator_cred = cred = prepare_creds();
> if (!cred)
> goto out_err;
> diff --git a/fs/overlayfs/util.c b/fs/overlayfs/util.c
> index d6bb1c9..4d0521a 100644
> --- a/fs/overlayfs/util.c
> +++ b/fs/overlayfs/util.c
> @@ -254,8 +254,14 @@ void ovl_dentry_set_redirect(struct dentry *dentry, const char *redirect)
> void ovl_inode_init(struct inode *inode, struct dentry *upperdentry,
> struct dentry *lowerdentry)
> {
> - if (upperdentry)
> + struct ovl_fs *ofs = inode->i_sb->s_fs_info;
> +
> + if (upperdentry) {
> OVL_I(inode)->__upperdentry = upperdentry;
> + spin_lock(&ofs->upper_inodes_lock);
> + list_add(&OVL_I(inode)->upper_inodes_list, &ofs->upper_inodes);
> + spin_unlock(&ofs->upper_inodes_lock);
> + }
I checked the callers of ovl_inode_init() and ovl_inode_update() and I believe
the code duplication in this patch can be avoided by changing the code to:
if (upperdentry)
- OVL_I(inode)->__upperdentry = upperdentry;
+ ovl_inode_update(inode, upperdentry);
After the patch "ovl: hash directory inodes for fsnotify" this change will cause
the root overlay dir to also be hashed from ovl_inode_init() call in
ovl_fill_super(),
but that is not a bad thing. It is even needed for NFS export anyway.
Anyway, if you do make this change, please submit it in a separate patch and if
you don't make this change, please create a small helper to reduce this code
duplication.
Thanks,
Amir.
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