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

[Amir Goldstein <amir73il@gmail.com>]

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.

Thanks,
Amir.