linux-kernel - Re: [PATCH v7 4/6] block: loop: prepare for supporing direct IO

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Date:	Tue, 28 Jul 2015 08:06:22 +1000
From:	Dave Chinner <david@...morbit.com>
To:	Ming Lei <ming.lei@...onical.com>
Cc:	Christoph Hellwig <hch@...radead.org>,
	Jens Axboe <axboe@...nel.dk>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"Justin M. Forbes" <jforbes@...oraproject.org>,
	Jeff Moyer <jmoyer@...hat.com>, Tejun Heo <tj@...nel.org>,
	linux-api@...r.kernel.org
Subject: Re: [PATCH v7 4/6] block: loop: prepare for supporing direct IO

On Mon, Jul 27, 2015 at 05:53:33AM -0400, Ming Lei wrote:
> On Mon, Jul 27, 2015 at 5:45 AM, Christoph Hellwig <hch@...radead.org> wrote:
> > On Mon, Jul 27, 2015 at 05:41:57AM -0400, Ming Lei wrote:
> >> > Why the hardcoded value?  I suspect this should be more like:
> >> >
> >> >         if (dio && inode->i_sb->s_bdev &&
> >> >             (lo->lo_offset & (bdev_io_min(inode->i_sb->s_bdev) - 1)) != 0)
> >> >                 dio = false;
> >>
> >> The above can't work if the backing device has a bigger sector size
> >> (such as 4K), that is why loop's direct-io requires 512 min_io_size of
> >> backing device.
> >
> > Why doesn't it work?  If the backing device sector size is 4k
> > and lo_offset is 0 or a multiple of 4k it should allow direct I/O,
> > and my code sniplet will allow that.
> 
> Because size has to be 4k aligned too.

So check that, too. Any >= 4k block size filesystem should be doing
mostly 4k aligned and sized IO...

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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