linux-kernel - Re: [PATCH v4 5/7] fs: prioritize and separate direct_io from dax

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Date:	Mon, 02 May 2016 22:22:10 +0300
From:	Boaz Harrosh <boaz@...xistor.com>
To:	Dan Williams <dan.j.williams@...el.com>
CC:	Vishal Verma <vishal.l.verma@...el.com>,
	"linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
	linux-block@...r.kernel.org, Jan Kara <jack@...e.cz>,
	Matthew Wilcox <matthew@....cx>,
	Dave Chinner <david@...morbit.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	XFS Developers <xfs@....sgi.com>, Jens Axboe <axboe@...com>,
	Linux MM <linux-mm@...ck.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Christoph Hellwig <hch@...radead.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-ext4 <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH v4 5/7] fs: prioritize and separate direct_io from dax_io

On 05/02/2016 09:48 PM, Dan Williams wrote:
<>
>> And then it keeps broken the aligned buffered writes, which are still
>> broken after this set.
> 
> ...identical to the current situation with a traditional disk.
> 

Not true!! please see what I wrote "aligned buffered writes"
If there are no reads involved then there are no errors returned
to application.

>> I have by now read the v2 patches. And I think you guys did not yet try
>> the proper fix for dax_do_io. I think you need to go deeper into the loops
>> and selectively call bdev_* when error on a specific page copy. No need to
>> go through direct_IO path at all.
> 
> We still reach a point where the minimum granularity of
> bdev_direct_access() is larger than a sector, so you end up still
> needing to have the application understand how to send a properly
> aligned I/O.  The semantics of how to send a properly aligned
> direct-I/O are already well understood, so we simply reuse that path.
> 

You are making a mountain out of a mouse. The simple copy of a file
from start (offset ZERO) to end-of-file which is the most common usage
on earth is perfectly aligned and needs not any O_DIRECT and is what is used
everywhere.

>> Do you need that I send you a patch to demonstrate what I mean?
> 
> I remain skeptical of what you are proposing, but yes, a patch has a
> better chance to move the discussion forward.
> 

Sigh! OK
Boaz