linux-ext4 - Re: [PATCH v4 5/7] fs: prioritize and separate direct_io from dax

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Message-ID: <CAPcyv4jWPTDbbw6uMFEEt2Kazgw+wb5Pfwroej--uQPE+AtUbA@mail.gmail.com>
Date:	Mon, 2 May 2016 09:01:58 -0700
From:	Dan Williams <dan.j.williams@...el.com>
To:	Boaz Harrosh <boaz@...xistor.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 Mon, May 2, 2016 at 8:41 AM, Boaz Harrosh <boaz@...xistor.com> wrote:
> On 04/29/2016 12:16 AM, Vishal Verma wrote:
>> All IO in a dax filesystem used to go through dax_do_io, which cannot
>> handle media errors, and thus cannot provide a recovery path that can
>> send a write through the driver to clear errors.
>>
>> Add a new iocb flag for DAX, and set it only for DAX mounts. In the IO
>> path for DAX filesystems, use the same direct_IO path for both DAX and
>> direct_io iocbs, but use the flags to identify when we are in O_DIRECT
>> mode vs non O_DIRECT with DAX, and for O_DIRECT, use the conventional
>> direct_IO path instead of DAX.
>>
>
> Really? What are your thinking here?
>
> What about all the current users of O_DIRECT, you have just made them
> 4 times slower and "less concurrent*" then "buffred io" users. Since
> direct_IO path will queue an IO request and all.
> (And if it is not so slow then why do we need dax_do_io at all? [Rhetorical])
>
> I hate it that you overload the semantics of a known and expected
> O_DIRECT flag, for special pmem quirks. This is an incompatible
> and unrelated overload of the semantics of O_DIRECT.

I think it is the opposite situation, it us undoing the premature
overloading of O_DIRECT that went in without performance numbers.
This implementation clarifies that dax_do_io() handles the lack of a
page cache for buffered I/O and O_DIRECT behaves as it nominally would
by sending an I/O to the driver.  It has the benefit of matching the
error semantics of a typical block device where a buffered write could
hit an error filling the page cache, but an O_DIRECT write potentially
triggers the drive to remap the block.
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