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

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Date:	Thu, 5 May 2016 21:39:14 +0000
From:	"Verma, Vishal L" <vishal.l.verma@...el.com>
To:	"hch@...radead.org" <hch@...radead.org>,
	"boaz@...xistor.com" <boaz@...xistor.com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
	"linux-nvdimm@...1.01.org" <linux-nvdimm@...1.01.org>,
	"xfs@....sgi.com" <xfs@....sgi.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"axboe@...com" <axboe@...com>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	"david@...morbit.com" <david@...morbit.com>,
	"jack@...e.cz" <jack@...e.cz>, "matthew@....cx" <matthew@....cx>
Subject: Re: [PATCH v4 5/7] fs: prioritize and separate direct_io from dax_io

On Thu, 2016-05-05 at 07:24 -0700, Christoph Hellwig wrote:
> On Mon, May 02, 2016 at 06:41:51PM +0300, Boaz Harrosh 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.
> Agreed - makig O_DIRECT less direct than not having it is plain
> stupid,
> and I somehow missed this initially.

How is it any 'less direct'? All it does now is follow the blockdev
O_DIRECT path. There still isn't any page cache involved..

> 
> This whole DAX story turns into a major nightmare, and I fear all our
> hodge podge tweaks to the semantics aren't helping it.
> 
> It seems like we simply need an explicit O_DAX for the read/write
> bypass if can't sort out the semantics (error, writer synchronization)
> just as we need a special flag for MMAP..