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

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Message-ID: <20091217175120.GA5741@infradead.org>
Date:	Thu, 17 Dec 2009 12:51:20 -0500
From:	Christoph Hellwig <hch@...radead.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Christoph Hellwig <hch@...radead.org>, tytso@....edu,
	Kyle McMartin <kyle@...artin.ca>, linux-parisc@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	James.Bottomley@...e.de, linux-arch@...r.kernel.org,
	Jens Axboe <jens.axboe@...cle.com>
Subject: Re: [git patches] xfs and block fixes for virtually indexed arches

On Thu, Dec 17, 2009 at 09:42:15AM -0800, Linus Torvalds wrote:
> > Which is exactly what the XFS code does.  Pages are allocated manually
> > and we store pointers to the page struct that later get added to the
> > bio.
> 
> Hmm. The BIO interface that the patch-series changes (bio_map_kern) 
> doesn't work that way. It takes a "buf, len" kind of thing. That's what 
> I'm complaining about.

Indeed, the "block: permit I/O to vmalloc/vmap kernel pages" does what
you complain about.  But the series doesn't actually add a user for
that.  What it does in XFS is quite a bit of black magic, too - but only
with the new cache coherence calls that are noops on architectures with
physically indexed caches.

> Well, they clearly are _after_ this series, since that's what all those 
> changes to __bio_map_kernel() and bio_map_kern_endio() are all about.
>
> So I believe you when you say that XFS perhaps does everything right - I 
> just think that the patch series in question actually makes things worse, 
> exactly because it is starting to use virtual addresses.

I'm not entirely sure why James added those, but XFS doesn't actually
use bio_map_kern.

> And I really think that would be all much more properly done at the 
> _caller_ level, not by the BIO layer.
>
> You must have some locking and allocation etc logic at the caller anyway, 
> why doesn't _that_ level just do the flushing or invalidation?

http://git.kernel.org/?p=linux/kernel/git/kyle/parisc-2.6.git;a=commitdiff;h=56c8214b842324e94aa88012010b0f1f9847daec

does it in the caller level.  Not exactly in a beautiful way, but who
am I complain as I'm already lost in our mess of cache coherency APIs.

> IOW, I'm perfectly happy with the patch to fs/xfs/linux-2.6/xfs_buf.c. 
> That one still seems to use 'bio_add_page()' with a regular 'struct page'. 
> 
> But the fs/bio.c patch looks like just total and utter crap to me, and is 
> the reason I refuse to pull this series.

Kyle/James, can you regenerate the tree without that patch included?

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