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Date: Sat, 8 Aug 2015 23:44:39 -0700
From: Christoph Hellwig <hch@infradead.org>
To: Ming Lei <ming.lei@canonical.com>
Cc: Christoph Hellwig <hch@infradead.org>, Jens Axboe <axboe@kernel.dk>,
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Subject: Re: [PATCH v9 6/6] block: loop: support DIO & AIO
Message-ID: <20150809064439.GC2436@infradead.org>
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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@infradead.org> 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.