Re: [PATCH 4/4] vfs: RWF_NONBLOCK flag for preadv2

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What would be required for a network file system to support the
RWF_NONBLOCK flag?

SMB3 operations are all async (on the wire) by default but they do
block on the network send and interim response.

On Tue, Oct 21, 2014 at 3:46 PM, Milosz Tanski <milosz@xxxxxxxxx> wrote:
> Filesystems that generic_file_read_iter will not be allowed to perform
> non-blocking reads. This only will read data if it's in the page cache and if
> there is no page error (causing a re-read).
>
> Christoph Hellwig wrote the filesystem specify code (cifs, ofs, shm, xfs).
>
> Signed-off-by: Milosz Tanski <milosz@xxxxxxxxx>
> ---
>  fs/cifs/file.c     |  6 ++++++
>  fs/ocfs2/file.c    |  6 ++++++
>  fs/pipe.c          |  3 ++-
>  fs/read_write.c    | 21 ++++++++++++++-------
>  fs/xfs/xfs_file.c  |  4 ++++
>  include/linux/fs.h |  3 +++
>  mm/filemap.c       | 18 ++++++++++++++++++
>  mm/shmem.c         |  4 ++++
>  8 files changed, 57 insertions(+), 8 deletions(-)
>
> diff --git a/fs/cifs/file.c b/fs/cifs/file.c
> index 3e4d00a..c485afa 100644
> --- a/fs/cifs/file.c
> +++ b/fs/cifs/file.c
> @@ -3005,6 +3005,9 @@ ssize_t cifs_user_readv(struct kiocb *iocb, struct iov_iter *to)
>         struct cifs_readdata *rdata, *tmp;
>         struct list_head rdata_list;
>
> +       if (iocb->ki_rwflags & RWF_NONBLOCK)
> +               return -EAGAIN;
> +
>         len = iov_iter_count(to);
>         if (!len)
>                 return 0;
> @@ -3123,6 +3126,9 @@ cifs_strict_readv(struct kiocb *iocb, struct iov_iter *to)
>             ((cifs_sb->mnt_cifs_flags & CIFS_MOUNT_NOPOSIXBRL) == 0))
>                 return generic_file_read_iter(iocb, to);
>
> +       if (iocb->ki_rwflags & RWF_NONBLOCK)
> +               return -EAGAIN;
> +
>         /*
>          * We need to hold the sem to be sure nobody modifies lock list
>          * with a brlock that prevents reading.
> diff --git a/fs/ocfs2/file.c b/fs/ocfs2/file.c
> index 324dc93..bb66ca4 100644
> --- a/fs/ocfs2/file.c
> +++ b/fs/ocfs2/file.c
> @@ -2472,6 +2472,12 @@ static ssize_t ocfs2_file_read_iter(struct kiocb *iocb,
>                         filp->f_path.dentry->d_name.name,
>                         to->nr_segs);   /* GRRRRR */
>
> +       /*
> +        * No non-blocking reads for ocfs2 for now.  Might be doable with
> +        * non-blocking cluster lock helpers.
> +        */
> +       if (iocb->ki_rwflags & RWF_NONBLOCK)
> +               return -EAGAIN;
>
>         if (!inode) {
>                 ret = -EINVAL;
> diff --git a/fs/pipe.c b/fs/pipe.c
> index 21981e5..212bf68 100644
> --- a/fs/pipe.c
> +++ b/fs/pipe.c
> @@ -302,7 +302,8 @@ pipe_read(struct kiocb *iocb, struct iov_iter *to)
>                          */
>                         if (ret)
>                                 break;
> -                       if (filp->f_flags & O_NONBLOCK) {
> +                       if ((filp->f_flags & O_NONBLOCK) ||
> +                           (iocb->ki_rwflags & RWF_NONBLOCK)) {
>                                 ret = -EAGAIN;
>                                 break;
>                         }
> diff --git a/fs/read_write.c b/fs/read_write.c
> index e3d8451..955d829 100644
> --- a/fs/read_write.c
> +++ b/fs/read_write.c
> @@ -835,14 +835,19 @@ static ssize_t do_readv_writev(int type, struct file *file,
>                 file_start_write(file);
>         }
>
> -       if (iter_fn)
> +       if (iter_fn) {
>                 ret = do_iter_readv_writev(file, type, iov, nr_segs, tot_len,
>                                                 pos, iter_fn, flags);
> -       else if (fnv)
> -               ret = do_sync_readv_writev(file, iov, nr_segs, tot_len,
> -                                               pos, fnv);
> -       else
> -               ret = do_loop_readv_writev(file, iov, nr_segs, pos, fn);
> +       } else {
> +               if (type == READ && (flags & RWF_NONBLOCK))
> +                       return -EAGAIN;
> +
> +               if (fnv)
> +                       ret = do_sync_readv_writev(file, iov, nr_segs, tot_len,
> +                                                       pos, fnv);
> +               else
> +                       ret = do_loop_readv_writev(file, iov, nr_segs, pos, fn);
> +       }
>
>         if (type != READ)
>                 file_end_write(file);
> @@ -866,8 +871,10 @@ ssize_t vfs_readv(struct file *file, const struct iovec __user *vec,
>                 return -EBADF;
>         if (!(file->f_mode & FMODE_CAN_READ))
>                 return -EINVAL;
> -       if (flags & ~0)
> +       if (flags & ~RWF_NONBLOCK)
>                 return -EINVAL;
> +       if ((file->f_flags & O_DIRECT) && (flags & RWF_NONBLOCK))
> +               return -EAGAIN;
>
>         return do_readv_writev(READ, file, vec, vlen, pos, flags);
>  }
> diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> index eb596b4..b1f6334 100644
> --- a/fs/xfs/xfs_file.c
> +++ b/fs/xfs/xfs_file.c
> @@ -246,6 +246,10 @@ xfs_file_read_iter(
>
>         XFS_STATS_INC(xs_read_calls);
>
> +       /* XXX: need a non-blocking iolock helper, shouldn't be too hard */
> +       if (iocb->ki_rwflags & RWF_NONBLOCK)
> +               return -EAGAIN;
> +
>         if (unlikely(file->f_flags & O_DIRECT))
>                 ioflags |= XFS_IO_ISDIRECT;
>         if (file->f_mode & FMODE_NOCMTIME)
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 9ed5711..eaebd99 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -1459,6 +1459,9 @@ struct block_device_operations;
>  #define HAVE_COMPAT_IOCTL 1
>  #define HAVE_UNLOCKED_IOCTL 1
>
> +/* These flags are used for the readv/writev syscalls with flags. */
> +#define RWF_NONBLOCK 0x00000001
> +
>  struct iov_iter;
>
>  struct file_operations {
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 45964c8..e73ba7e 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -1493,6 +1493,8 @@ static ssize_t do_generic_file_read(struct file *filp, loff_t *ppos,
>  find_page:
>                 page = find_get_page(mapping, index);
>                 if (!page) {
> +                       if (flags & RWF_NONBLOCK)
> +                               goto would_block;
>                         page_cache_sync_readahead(mapping,
>                                         ra, filp,
>                                         index, last_index - index);
> @@ -1584,6 +1586,11 @@ page_ok:
>                 continue;
>
>  page_not_up_to_date:
> +               if (flags & RWF_NONBLOCK) {
> +                       page_cache_release(page);
> +                       goto would_block;
> +               }
> +
>                 /* Get exclusive access to the page ... */
>                 error = lock_page_killable(page);
>                 if (unlikely(error))
> @@ -1603,6 +1610,12 @@ page_not_up_to_date_locked:
>                         goto page_ok;
>                 }
>
> +               if (flags & RWF_NONBLOCK) {
> +                       unlock_page(page);
> +                       page_cache_release(page);
> +                       goto would_block;
> +               }
> +
>  readpage:
>                 /*
>                  * A previous I/O error may have been due to temporary
> @@ -1673,6 +1686,8 @@ no_cached_page:
>                 goto readpage;
>         }
>
> +would_block:
> +       error = -EAGAIN;
>  out:
>         ra->prev_pos = prev_index;
>         ra->prev_pos <<= PAGE_CACHE_SHIFT;
> @@ -1706,6 +1721,9 @@ generic_file_read_iter(struct kiocb *iocb, struct iov_iter *iter)
>                 size_t count = iov_iter_count(iter);
>                 loff_t size;
>
> +               if (iocb->ki_rwflags & RWF_NONBLOCK)
> +                       return -EAGAIN;
> +
>                 if (!count)
>                         goto out; /* skip atime */
>                 size = i_size_read(inode);
> diff --git a/mm/shmem.c b/mm/shmem.c
> index cd6fc75..5c30f04 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -1531,6 +1531,10 @@ static ssize_t shmem_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
>         ssize_t retval = 0;
>         loff_t *ppos = &iocb->ki_pos;
>
> +       /* XXX: should be easily supportable */
> +       if (iocb->ki_rwflags & RWF_NONBLOCK)
> +               return -EAGAIN;
> +
>         /*
>          * Might this read be for a stacking filesystem?  Then when reading
>          * holes of a sparse file, we actually need to allocate those pages,
> --
> 1.9.1
>
> --
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-- 
Thanks,

Steve
--
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