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

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

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.

>> +               if (ret != -EIOCBQUEUED) {
>> +                       iocb->ki_pos = aio_req->iocb.ki_pos;
>> +                       fdput(real);
>> +                       kmem_cache_free(ovl_aio_request_cachep, aio_req);
>> +               }
> 
> Suggest cleanup helper for completion/error
> 
Yes, that will be more clearly. I will add a cleanup helper in version 2.

> 
>> +       }
>>         revert_creds(old_cred);
>>
>> -       ovl_file_accessed(file);
>> -
>> -       fdput(real);
>> -
>>         return ret;
>>  }
>>
>> @@ -275,16 +312,32 @@ static ssize_t ovl_write_iter(struct kiocb *iocb, struct iov_iter *iter)
>>
>>         old_cred = ovl_override_creds(file_inode(file)->i_sb);
>>         file_start_write(real.file);
>> -       ret = vfs_iter_write(real.file, iter, &iocb->ki_pos,
>> -                            ovl_iocb_to_rwf(iocb));
>> -       file_end_write(real.file);
>> +       if (is_sync_kiocb(iocb)) {
>> +               ret = vfs_iter_write(real.file, iter, &iocb->ki_pos,
>> +                                    ovl_iocb_to_rwf(iocb));
>> +               file_end_write(real.file);
>> +               /* Update size */
>> +               ovl_copyattr(ovl_inode_real(inode), inode);
>> +               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_write(real.file, &aio_req->iocb, iter);
>> +               /* Update size */
>> +               ovl_copyattr(ovl_inode_real(inode), inode);
> 
> That should be in completion/error
> 
Yes, I will move it to the newly added cleanup helper.

Thanks,
Jiufei

> Thanks,
> Amir.
>