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

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

On Thu, Apr 10, 2025 at 09:34:39AM +0200, Christoph Hellwig wrote:
> On Wed, Apr 09, 2025 at 09:44:41PM +0800, Ming Lei wrote:
> > 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.
> 
> LOOP_SET_STATUS changes the direct I/O flag.

It isn't true in the following test case.

> 
> This is the minimal reproducer, dev needs to be a 4k lba size device:
> 
> dev=/dev/nvme0n1
> 
> mkfs.xfs -f $dev
> mount $dev /mnt
> 
> truncate -s 30g /mnt/a
> losetup --direct-io=on -f --show /mnt/a
> losetup --direct-io=off /dev/loop0

direct I/O flag is changed by LOOP_SET_DIRECT_IO instead of LOOP_SET_STATUS.

losetup forgets the backfile is opened as O_DIRECT, and util-linux
need fix too, such as call F_GETFL/F_SETFL to clear O_DIRECT.

I guess it is hard to figure out one simple kernel fix to clear IOCB_DIRECT of
file->f_iocb_flags for backporting.


Thanks, 
Ming