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


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