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

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Date:	Sun, 8 May 2016 18:42:37 +0000
From:	"Verma, Vishal L" <vishal.l.verma@...el.com>
To:	"hch@...radead.org" <hch@...radead.org>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
	"xfs@....sgi.com" <xfs@....sgi.com>,
	"linux-nvdimm@...1.01.org" <linux-nvdimm@...1.01.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
	"Williams, Dan J" <dan.j.williams@...el.com>,
	"axboe@...com" <axboe@...com>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"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 Sun, 2016-05-08 at 02:01 -0700, hch@...radead.org wrote:
> On Thu, May 05, 2016 at 09:45:07PM +0000, Verma, Vishal L wrote:
> > 
> > I'm not sure I completely understand how this will work? Can you
> > explain
> > a bit? Would we have to export rw_bytes up to layers above the pmem
> > driver? Where does get_user_pages come in?
> A DAX filesystem can directly use the nvdimm layer the same way btt
> doe,s what's the problem?

The BTT does rw_bytes through an internal-to-libnvdimm mechanism, but
rw_bytes isn't exported to the filesystem, currently.. To do this we'd
have to either add an rw_bytes to block device operations...or
something.

Another thing is rw_bytes currently doesn't do error clearing either.
We store badblocks at sector granularity, and like Dan said earlier,
that hides the clear_error alignment requirements and upper layers
don't have to be aware of it. To make rw_bytes clear sub-sector errors,
we'd have to change the granularity of bad-blocks, and make upper
layers aware of the clearing alignment requirements.

Using a block-write semantic for clearing hides all this away.

> 
> Re get_user_pages my idea was to simply use that to lock down the
> user
> pages so that we can call rw_bytes on it.  How else would you do
> it?  Do
> a kmalloc, copy_from_user and then another memcpy?