Re: [PATCH] loop: stop using vfs_iter_{read,write} for buffered I/O

[Ming Lei <ming.lei@redhat.com>]

On Wed, Apr 09, 2025 at 03:09:40PM +0200, Christoph Hellwig wrote:
> vfs_iter_{read,write} always perform direct I/O when the file has the
> O_DIRECT flag set, which breaks disabling direct I/O using the
> LOOP_SET_STATUS / LOOP_SET_STATUS64 ioctls.

So dio is disabled automatically because lo_offset is changed in
LOOP_SET_STATUS, but backing file is still opened with O_DIRECT,
then dio fails?

But Darrick reports it is caused by changing sector size, instead of
LOOP_SET_STATUS.

> 
> This was recenly reported as a regression, but as far as I can tell
> was only uncovered by better checking for block sizes and has been
> around since the direct I/O support was added.

What is the 1st real bad commit for this regression? I think it is useful
for backporting. Or it is new test case?

> 
> Fix this by using the existing aio code that calls the raw read/write
> iter methods instead.  Note that despite the comments there is no need
> for block drivers to ever call flush_dcache_page themselves, and the
> call is a left-over from prehistoric times.
> 
> Fixes: ab1cb278bc70 ("block: loop: introduce ioctl command of LOOP_SET_DIRECT_IO")

Why is the issue related with ioctl(LOOP_SET_DIRECT_IO)?


Thanks, 
Ming