Date: Fri, 18 Dec 2009 13:44:40 +1100
From: Dave Chinner <david@fromorbit.com>
To: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Cc: James.Bottomley@suse.de, jens.axboe@oracle.com,
       torvalds@linux-foundation.org, tytso@mit.edu, kyle@mcmartin.ca,
       linux-parisc@vger.kernel.org, linux-kernel@vger.kernel.org,
       hch@infradead.org, linux-arch@vger.kernel.org
Subject: Re: [git patches] xfs and block fixes for virtually indexed arches
Message-ID: <20091218024440.GG4850@discord.disaster>
References: <alpine.LFD.2.00.0912170950330.15740@localhost.localdomain> <20091217193648.GI4489@kernel.dk> <1261094220.2752.27.camel@mulgrave.site> <20091218095944G.fujita.tomonori@lab.ntt.co.jp>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20091218095944G.fujita.tomonori@lab.ntt.co.jp>
User-Agent: Mutt/1.5.18 (2008-05-17)
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 3514
Lines: 85

On Fri, Dec 18, 2009 at 10:00:21AM +0900, FUJITA Tomonori wrote:
> On Fri, 18 Dec 2009 00:57:00 +0100
> James Bottomley <James.Bottomley@suse.de> wrote:
> 
> > On Thu, 2009-12-17 at 20:36 +0100, Jens Axboe wrote:
> > > On Thu, Dec 17 2009, Linus Torvalds wrote:
> > > > 
> > > > 
> > > > On Thu, 17 Dec 2009, tytso@mit.edu wrote:
> > > > > 
> > > > > Sure, but there's some rumors/oral traditions going around that some
> > > > > block devices want bio address which are page aligned, because they
> > > > > want to play some kind of refcounting game,
> > > > 
> > > > Yeah, you might be right at that.
> > > > 
> > > > > And it's Weird Shit(tm) (aka iSCSI, AoE) type drivers, that most of us 
> > > > > don't have access to, so just because it works Just Fine on SATA doesn't 
> > > > > mean anything.
> > > > > 
> > > > > And none of this is documented anywhere, which is frustrating as hell.
> > > > > Just rumors that "if you do this, AoE/iSCSI will corrupt your file
> > > > > systems".
> > > > 
> > > > ACK. Jens? 
> > > 
> > > I've heard those rumours too, and I don't even know if they are true.
> > > Who has a pointer to such a bug report and/or issue? The block layer
> > > itself doesn't not have any such requirements, and the only places where
> > > we play page games is for bio's that were explicitly mapped with pages
> > > by itself (like mapping user data).o
> > 
> > OK, so what happened is that prior to the map single fix
> > 
> > commit df46b9a44ceb5af2ea2351ce8e28ae7bd840b00f
> > Author: Mike Christie  <michaelc@cs.wisc.edu>
> > Date:   Mon Jun 20 14:04:44 2005 +0200
> > 
> >     [PATCH] Add blk_rq_map_kern()
> > 
> > 
> > bio could only accept user space buffers, so we had a special path for
> > kernel allocated buffers.  That commit unified the path (with a separate
> > block API) so we could now submit kmalloc'd buffers via block APIs.
> > 
> > So the rule now is we can accept any user mapped area via
> > blk_rq_map_user and any kmalloc'd area via blk_rq_map_kern().  We might
> > not be able to do a stack area (depending on how the arch maps the
> > stack) and we definitely cannot do a vmalloc'd area.
> > 
> > So it sounds like we only need a blk_rq_map_vmalloc() using the same
> > techniques as the patch set and we're good to go.
> 
> I'm not sure about it.
> 
> As I said before (when I was against this 'adding vmalloc support to
> the block layer' stuff), are there potential users of this except for
> XFS? Are there anyone who does such a thing now?

As Christoph already mentioned, XFS is not passing the vmalloc'd
range to the block layer - it passes the underlying pages to the
block layer. Hence I'm not sure there actually is anyone who is
passing vmalloc'd addresses to the block layer. Perhaps we should
put a WARN_ON() in the block layer to catch anyone doing such a
thing before considering supporting vmalloc'd addresses in the block
layer?

> This API might be useful for only journaling file systems using log
> formats that need large contiguous buffer. Sound like only XFS?

FWIW, mapped buffers larger than PAGE_SIZE are used for more than just log
recovery in XFS. e.g. filesystems with directory block size larger
than page size uses mapped buffers.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/