Re: [PATCH 2/2] ovl: implement async IO routines

[Jiufei Xue <jiufei.xue@linux.alibaba.com>]

Hi Amir,
On 2019/11/19 下午5:38, Amir Goldstein wrote:
> On Tue, Nov 19, 2019 at 10:37 AM Jiufei Xue
> <jiufei.xue@linux.alibaba.com> wrote:
>>
>> Hi Amir,
>>
>> On 2019/11/19 下午12:22, Amir Goldstein wrote:
>>> On Tue, Nov 19, 2019 at 4:14 AM Jiufei Xue <jiufei.xue@linux.alibaba.com> wrote:
>>>>
>>>> A performance regression is observed since linux v4.19 when we do aio
>>>> test using fio with iodepth 128 on overlayfs. And we found that queue
>>>> depth of the device is always 1 which is unexpected.
>>>>
>>>> After investigation, it is found that commit 16914e6fc7
>>>> (“ovl: add ovl_read_iter()”) and commit 2a92e07edc
>>>> (“ovl: add ovl_write_iter()”) use do_iter_readv_writev() to submit
>>>> requests to real filesystem. Async IOs are converted to sync IOs here
>>>> and cause performance regression.
>>>>
>>>> So implement async IO for stacked reading and writing.
>>>>
>>>> Signed-off-by: Jiufei Xue <jiufei.xue@linux.alibaba.com>
>>>> ---
>>>>  fs/overlayfs/file.c      | 97 +++++++++++++++++++++++++++++++++++++++++-------
>>>>  fs/overlayfs/overlayfs.h |  2 +
>>>>  fs/overlayfs/super.c     | 12 +++++-
>>>>  3 files changed, 95 insertions(+), 16 deletions(-)
>>>>
>>>> diff --git a/fs/overlayfs/file.c b/fs/overlayfs/file.c
>>>> index e235a63..07d94e7 100644
>>>> --- a/fs/overlayfs/file.c
>>>> +++ b/fs/overlayfs/file.c
>>>> @@ -11,6 +11,14 @@
>>>>  #include <linux/uaccess.h>
>>>>  #include "overlayfs.h"
>>>>
>>>> +struct ovl_aio_req {
>>>> +       struct kiocb iocb;
>>>> +       struct kiocb *orig_iocb;
>>>> +       struct fd fd;
>>>> +};
>>>> +
>>>> +static struct kmem_cache *ovl_aio_request_cachep;
>>>> +
>>>>  static char ovl_whatisit(struct inode *inode, struct inode *realinode)
>>>>  {
>>>>         if (realinode != ovl_inode_upper(inode))
>>>> @@ -225,6 +233,21 @@ static rwf_t ovl_iocb_to_rwf(struct kiocb *iocb)
>>>>         return flags;
>>>>  }
>>>>
>>>> +static void ovl_aio_rw_complete(struct kiocb *iocb, long res, long res2)
>>>> +{
>>>> +       struct ovl_aio_req *aio_req = container_of(iocb, struct ovl_aio_req, iocb);
>>>> +       struct kiocb *orig_iocb = aio_req->orig_iocb;
>>>> +
>>>> +       if (iocb->ki_flags & IOCB_WRITE)
>>>> +               file_end_write(iocb->ki_filp);
>>>> +
>>>> +       orig_iocb->ki_pos = iocb->ki_pos;
>>>> +       orig_iocb->ki_complete(orig_iocb, res, res2);
>>>> +
>>>> +       fdput(aio_req->fd);
>>>> +       kmem_cache_free(ovl_aio_request_cachep, aio_req);
>>>> +}
>>>> +
>>>>  static ssize_t ovl_read_iter(struct kiocb *iocb, struct iov_iter *iter)
>>>>  {
>>>>         struct file *file = iocb->ki_filp;
>>>> @@ -240,14 +263,28 @@ static ssize_t ovl_read_iter(struct kiocb *iocb, struct iov_iter *iter)
>>>>                 return ret;
>>>>
>>>>         old_cred = ovl_override_creds(file_inode(file)->i_sb);
>>>> -       ret = vfs_iter_read(real.file, iter, &iocb->ki_pos,
>>>> -                           ovl_iocb_to_rwf(iocb));
>>>> +       if (is_sync_kiocb(iocb)) {
>>>> +               ret = vfs_iter_read(real.file, iter, &iocb->ki_pos,
>>>> +                                   ovl_iocb_to_rwf(iocb));
>>>> +               ovl_file_accessed(file);
>>>> +               fdput(real);
>>>> +       } else {
>>>> +               struct ovl_aio_req *aio_req = kmem_cache_alloc(ovl_aio_request_cachep,
>>>> +                                                              GFP_NOFS);
>>>> +               aio_req->fd = real;
>>>> +               aio_req->orig_iocb = iocb;
>>>> +               kiocb_clone(&aio_req->iocb, iocb, real.file);
>>>> +               aio_req->iocb.ki_complete = ovl_aio_rw_complete;
>>>> +               ret = vfs_iocb_iter_read(real.file, &aio_req->iocb, iter);
>>>> +               ovl_file_accessed(file);
>>>
>>> That should be done in completion/error
>>>
>>
>> Refer to function generic_file_read_iter(), in direct IO path,
>> file_accessed() is done before IO submission, so I think ovl_file_accessed()
>> should be done here no matter completion/error or IO is queued.
> 
> Mmm, it doesn't matter much if atime is updated before or after,
> but ovl_file_accessed() does not only update atime, it also copies
> ctime which could have been modified as a result of the io, so
> I think it is safer to put it in the cleanup hook.
>

Can you give a more detailed description that a read op will modify
ctime as a result of the io?

I found that it will trigger BUG_ON(irqs_disabled()) while
calling ovl_file_accessed() on async IO return path. The calltrace
is pasted below:

ovl_file_accessed
  -> touch_atime
    -> ovl_update_time
      -> generic_update_time
        -> __mark_inode_dirty
          -> ext4_dirty_inode
            -> __ext4_get_inode_loc
              -> __find_get_block
                -> lookup_bh_lru
                   -> check_irqs_on

So I need more detail to find how to fix this issue.

Thanks,
Jiufei.

> Thanks,
> Amir.
>