linux-kernel - Re: [RFC PATCH] fuse: modification of FUSE passthrough call sequence

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Message-ID: <CAOQ4uxi5gwzkEYqpd+Bb825jwWME_AE0BNykZcownSz6OZjFWQ@mail.gmail.com>
Date: Wed, 16 Jul 2025 14:14:49 +0200
From: Amir Goldstein <amir73il@...il.com>
To: Qi Han <hanqi@...o.com>
Cc: miklos@...redi.hu, linux-fsdevel@...r.kernel.org, 
	linux-kernel@...r.kernel.org, liulei.rjpt@...o.com
Subject: Re: [RFC PATCH] fuse: modification of FUSE passthrough call sequence

On Wed, Jul 16, 2025 at 1:49 PM Qi Han <hanqi@...o.com> wrote:
>
> Hi, Amir

Hi Qi,

> In the commit [1], performing read/write operations with DIRECT_IO on
> a FUSE file path does not trigger FUSE passthrough. I am unclear about
> the reason behind this behavior. Is it possible to modify the call
> sequence to support passthrough for files opened with DIRECT_IO?

Are you talking about files opened by user with O_DIRECT or
files open by server with FOPEN_DIRECT_IO?

Those are two different things.
IIRC, O_DIRECT to a backing passthrough file should be possible.

> Thank you!
>
> [1]
> https://lore.kernel.org/all/20240206142453.1906268-7-amir73il@gmail.com/
>
> Reported-by: Lei Liu <liulei.rjpt@...o.com>
> Signed-off-by: Qi Han <hanqi@...o.com>
> ---
>  fs/fuse/file.c | 15 +++++++--------
>  1 file changed, 7 insertions(+), 8 deletions(-)
>
> diff --git a/fs/fuse/file.c b/fs/fuse/file.c
> index 2ddfb3bb6483..689f9ee938f1 100644
> --- a/fs/fuse/file.c
> +++ b/fs/fuse/file.c
> @@ -1711,11 +1711,11 @@ static ssize_t fuse_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
>         if (FUSE_IS_DAX(inode))
>                 return fuse_dax_read_iter(iocb, to);
>
> -       /* FOPEN_DIRECT_IO overrides FOPEN_PASSTHROUGH */
> -       if (ff->open_flags & FOPEN_DIRECT_IO)
> -               return fuse_direct_read_iter(iocb, to);
> -       else if (fuse_file_passthrough(ff))
> +
> +       if (fuse_file_passthrough(ff))
>                 return fuse_passthrough_read_iter(iocb, to);
> +       else if (ff->open_flags & FOPEN_DIRECT_IO)
> +               return fuse_direct_read_iter(iocb, to);
>         else
>                 return fuse_cache_read_iter(iocb, to);
>  }
> @@ -1732,11 +1732,10 @@ static ssize_t fuse_file_write_iter(struct kiocb *iocb, struct iov_iter *from)
>         if (FUSE_IS_DAX(inode))
>                 return fuse_dax_write_iter(iocb, from);
>
> -       /* FOPEN_DIRECT_IO overrides FOPEN_PASSTHROUGH */
> -       if (ff->open_flags & FOPEN_DIRECT_IO)
> -               return fuse_direct_write_iter(iocb, from);
> -       else if (fuse_file_passthrough(ff))
> +       if (fuse_file_passthrough(ff))
>                 return fuse_passthrough_write_iter(iocb, from);
> +       else if (ff->open_flags & FOPEN_DIRECT_IO)
> +               return fuse_direct_write_iter(iocb, from);
>         else
>                 return fuse_cache_write_iter(iocb, from);
>  }
> --

When server requests to open a file with FOPEN_DIRECT_IO,
it affects how FUSE_READ/FUSE_WRITE requests are made.

When server requests to open a file with FOPEN_PASSTHROUGH,
it means that FUSE_READ/FUSE_WRITE requests are not to be
expected at all, so these two options are somewhat conflicting.

Therefore, I do not know what you aim to achieve by your patch.

However, please note this comment in iomode.c:
 * A combination of FOPEN_PASSTHROUGH and FOPEN_DIRECT_IO
   means that read/write
 * operations go directly to the server, but mmap is done on the backing file.

So this is a special mode that the server can request in order to do
passthrough mmap but still send FUSE_READ/FUSE_WRITE requests
to the server.

What is your use case? What are you trying to achieve that is not
currently possible?

Thanks,
Amir.