linux-kernel - Re: [git patches] xfs and block fixes for virtually indexed arches

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Date:	Fri, 18 Dec 2009 00:57:00 +0100
From:	James Bottomley <James.Bottomley@...e.de>
To:	Jens Axboe <jens.axboe@...cle.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>, tytso@....edu,
	Kyle McMartin <kyle@...artin.ca>, linux-parisc@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	hch@...radead.org, linux-arch@...r.kernel.org
Subject: Re: [git patches] xfs and block fixes for virtually indexed arches

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@....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@...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.

> We fix driver crap like that, we don't work around it. It's a BUG.

James



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