> 在 2018年1月9日，下午9:29，Amir Goldstein <amir73il@xxxxxxxxx> 写道：
>
> On Tue, Jan 9, 2018 at 2:07 PM, Chengguang Xu <cgxu519@xxxxxxxxxx> wrote:
>>>
>>> 在 2018年1月8日，下午8:10，Amir Goldstein <amir73il@xxxxxxxxx> 写道：
>>>
>>> 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…
>>
>> IIUC, when wait == 1 should use WB_SYNC_ALL mode.
>
> Well, yes, but for a different reason.
> According to documentation of write_cache_pages()
> WB_SYNC_ALL should be used for data integrity sync, which
> is the case of syncfs, to start write also on pages that are already
> under io, but WB_SYNC_ALL *also* implies waiting on all pages
> after starting io and that is not needed in this case, because you call
> filemap_fdatawait_keep_errors() later. Nevermind. WB_SYNC_ALL
> is the right value to use here AFAICT.
If you look carefully inside __writeback_single_inode(), there is a condition
to check whether this writeback caused by reason “for_sync" or not, and if it is
then skip wait because it has a separate, external IO completion path and sync_fs
for guaranteeing inode metadata is written back correctly.
>
>>
>>
>>>
>>>> + .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???
>>
>> I think if hold s_inode_list_lock long time, it will probably affect
>> inode allocation/destruction because every inode needs to be in/out
>> sb->s_inodes list.
>>
>>
>
> Considering that we must use WB_SYNC_ALL, my comment here
> is irrelevant.
>
>>> Do we not need to check return value from sync_inode()?
>>
>> I have no idea what extra work can we do for error case,
>> __sync_filesytem() itself does not check return code seriously.
>>
>>
>>> I bet it could be EROFS if upper fs was remounted readonly
>>> and we did not check for that before starting to sync…
>>
>> I’m curious is there a real use case like this?
>> Anyway, let’s add check for safety.
>>
>>>
>>>> + 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.
>>
>> I can’t see benefit for it, and considering interruption by signal
>> I’ll replace temporary waiting list using sb->s_inodes list.
>>
>
> If you use mutex_lock_interruptible() at least then the 2nd user calling
> syncfs can interrupt the wait while 1st user is syncing.
Current syncfs for other filesystems does not behave like this,
so I want to keep consistency with others.
Thanks,
Chengguang.
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