Re: [PATCH 1/2] fs: add FMODE_DIO_PARALLEL_WRITE flag

[Miklos Szeredi <miklos@xxxxxxxxxx>]

On Wed, 12 Apr 2023 at 15:42, Bernd Schubert <bschubert@xxxxxxx> wrote:
>
> Miklos, Jens,
>
> could we please also set this flag for fuse?
>
>
> Thanks,
> Bernd
>
>
> fuse: Set FMODE_DIO_PARALLEL_WRITE flag
>
> From: Bernd Schubert <bschubert@xxxxxxx>
>
> Fuse can also do parallel DIO writes, if userspace has enabled it.
>
> Signed-off-by: Bernd Schubert <bschubert@xxxxxxx>
> ---
>  fs/fuse/file.c |    3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/fs/fuse/file.c b/fs/fuse/file.c
> index 875314ee6f59..46e7f1196fd1 100644
> --- a/fs/fuse/file.c
> +++ b/fs/fuse/file.c
> @@ -215,6 +215,9 @@ void fuse_finish_open(struct inode *inode, struct file *file)
>         }
>         if ((file->f_mode & FMODE_WRITE) && fc->writeback_cache)
>                 fuse_link_write_file(file);
> +
> +       if (ff->open_flags & FOPEN_PARALLEL_DIRECT_WRITES)
> +               file->f_mode |= FMODE_DIO_PARALLEL_WRITE;

fuse_direct_write_iter():

bool exclusive_lock =
    !(ff->open_flags & FOPEN_PARALLEL_DIRECT_WRITES) ||
    iocb->ki_flags & IOCB_APPEND ||
    fuse_direct_write_extending_i_size(iocb, from);

If the write is size extending, then it will take the lock exclusive.
OTOH, I guess that it would be unusual for lots of  size extending
writes to be done in parallel.

What would be the effect of giving the  FMODE_DIO_PARALLEL_WRITE hint
and then still serializing the writes?

Thanks,
Miklos