Re: [PATCH v9 6/6] block: loop: support DIO & AIO

From: Christoph Hellwig
Date: Sun Aug 09 2015 - 02:44:47 EST

On Fri, Aug 07, 2015 at 04:25:52AM -0400, Ming Lei wrote:
> On Fri, Aug 7, 2015 at 3:43 AM, Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote:
> > I really disagree with the per-cmd use_dio tracking.
> Could you explain it in a bit?
> >
> > If we know at setup time that the loop device sector size is smaller
> > than the sector size of the underlying device we should never allow
> > dio, and othewise it should always work for data.
> Yes, that is just what I did in v7, and we can only do dio in case
> of 512 byte sector size of backing device(not considering the
> following patches from Hannes).
> When sector size of backing device isn't 512, most of transfer(buffered I/O
> and normal dio) is still 4k aligned, that is why I suggest to use per-cmd
> use_dio tracking.
> The patch avoids the race between buffered io and dio, doesn't it?
> The introduced cost is trivial and most of times it needn't to wait for
> completion of pending dio.

All block filesystems can do direct I/O on a _sector size_, not
_block size_ boundary, e.g. for the typical setup of a 4k block size
xfs/btrfs/ext4 file system on a 512 byte sector device you can do 512
byte aligned direct I/O.

> > is no need for draining or mode checking for an fsync - FLUSH is always
> > only guranteed to flush out I/O that has completed by the time it's
> > issued.
> Could you point it out in the patch?

Basically your lo_drain_pending_dio() functionality is not needed.
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