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

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Message-ID: <bd44135e-a86f-4556-8219-baa2f73c98c9@vivo.com>
Date: Thu, 17 Jul 2025 10:23:40 +0800
From: hanqi <hanqi@...o.com>
To: Amir Goldstein <amir73il@...il.com>, Ed.Tsai@...iatek.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



在 2025/7/16 20:14, Amir Goldstein 写道:
> 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.

Hi, Amir
Thank you for your response. I am performing read/write operations on
a file under a FUSE path opened with O_DIRECT, using code similar to [1].
However, it seems that the FUSE daemon adds FOPEN_DIRECT_IO, as Ed
mentioned. I need to further investigate the FUSE daemon code to confirm
the reason behind this behavior.

[1]
fd_in = open(src_path, O_RDONLY | O_DIRECT);
fd_out = open(dst_path, O_WRONLY | O_CREAT | O_TRUNC | O_DIRECT, 0644);

Thanks,
Qi.

>> 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.
>