Re: [PATCH] fuse: remove useless IOCB_DIRECT in fuse_direct_read/write_iter

[yangyun <yangyun50@huawei.com>]

On Tue, Aug 27, 2024 at 04:30:04PM +0800, Jingbo Xu wrote:
> Hi Miklos,
> 
> On 8/27/24 3:12 AM, Miklos Szeredi wrote:
> > On Mon, 26 Aug 2024 at 15:07, yangyun <yangyun50@huawei.com> wrote:
> >>
> >> Commit 23c94e1cdcbf ("fuse: Switch to using async direct IO
> >> for FOPEN_DIRECT_IO") gave the async direct IO code path in the
> >> fuse_direct_read_iter() and fuse_direct_write_iter(). But since
> >> these two functions are only called under FOPEN_DIRECT_IO is set,
> >> it seems that we can also use the async direct IO even the flag
> >> IOCB_DIRECT is not set to enjoy the async direct IO method. Also
> >> move the definition of fuse_io_priv to where it is used in fuse_
> >> direct_write_iter.
> > 
> > I'm interested in the motivation for this patch.
> > 
> > There's a minor risk of regressions when introducing such a behavior
> > change, so there should also be a strong supporting argument, which
> > seems to be missing in this case.
> > 
> 
> 
> I'm not sure what yangyun's use case is, but we indeed also observed a
> potential performance optimization for FOPEN_DIRECT_IO path.  When the
> buffer IO is submitted to a file flagged with FOPEN_DIRECT_IO, the code
> path is like:
> 
> fuse_direct_read_iter
>   __fuse_direct_read
>     fuse_direct_io
>       # split the request to multiple fuse requests according to
>       # max_read and max_pages constraint, for each split request:
>         fuse_send_read
>           fuse_simple_request
> 
> When the size of the user requested IO is greater than max_read and
> max_pages constraint, it's split into multiple requests and these split
> requests can not be sent to the fuse server until the previous split
> request *completes* (since fuse_simple_request()), even when the user
> request is submitted from async IO e.g. io-uring.

The same use case. Your explanation is more explicit.

And I just don't know why commit 23c94e1cdcbf ("fuse: Switch to using async 
direct IO for FOPEN_DIRECT_IO") adds the check of IOCB_DIRECT flag when using 
async direct_io. It seems unnessary.

> 
> -- 
> Thanks,
> Jingbo