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

From: Linus Torvalds
Date: Thu Dec 17 2009 - 12:43:20 EST

On Thu, 17 Dec 2009, Christoph Hellwig wrote:
> On Thu, Dec 17, 2009 at 08:46:33AM -0800, Linus Torvalds wrote:
> > The whole "vmalloc is special" has always been true. If you want to
> > treat vmalloc as normal memory, you need to look up the pages yourself. We
> > have helpers for that (including helpers that populate vmalloc space from
> > a page array to begin with - so you can _start_ from some array of pages
> > and then lay them out virtually if you want to have a convenient CPU
> > access to the array).
> 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.

> But we access them through vmap (which I added exactly for this
> reason back in 2002) for kernel accesses. On all architectures with
> sane caches things just work, but for parisc, arm and friends that have
> virtually indexed caches we need to make sure to flush caches for this
> different access. The vmalloc linear address is not used for I/O
> everywhere.

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 also think that the changes to bio_map_kernel() and bio_map_kern_endio()
are not just "fundamentally ugly", I think they are made worse by the fact
that it's not even done "right". You both flush the virtual caches before
the IO and invalidate after - when the real pattern should be that you
flush it before a write, and invalidate it after a read.

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?

I get the feeling that somebody decided that the whole "do DMA to/from
vmalloc space" was somehow a common generic pattern that should be
supported in general, and I violently disagree. Maybe XFS has good reasons
for doing it, but that does emphatically _not_ make it a good idea in
general, and that does _not_ mean that the BIO layer should make it easy
to do for other users and have a general interface for that kind of

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.

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