Re: [patch] loop.c to use write ops for fs requiring special locking

[Andrew Morton <akpm@osdl.org>]

Robert S Peterson <rpeterso@redhat.com> wrote:
>
> Hi All,
> 
> Below is a small patch to loop.c I'd like to see in the kernel.
> 
> This is an extension of Anton Altaparmakov's previous fix which allows
> loop.c to use the aop->write rather than prepare_write/commit_write if
> prepare_write/commit_write aren't available.

There's a reason why we must use prepare_write/commit_write rather than
->write() and ow-ow-my-brain-hurts I can't remember what it was.  Anton, do
you recall?

> Right now, the current loop.c uses aop->prepare_write/commit_write
> unless there is no other option.  However, due to special locking
> requirements, some backing filesystems may prefer the use of aop->write
> rather than prepare_write/commit_write.  Since loop.c does not have
> advisory locking, the backing fs should have a choice of which to use.
> 
> In the case of GFS, for example, loop.c's use of aop->prepare_write
> circumvents proper cluster locking and transaction building, so using
> aop->write is the right thing for loop.c to do.
> 
> How the patch works:
> If the backing filesystem has special locking requirements (new flag in
> fs_flags) loop.c uses aop->write rather than prepare_write/commit_write.
> 

I think you'll find that cryptoloop doesn't work (see "ow-ow", above).

> --- linux-2.6.15.4.orig/drivers/block/loop.c    2006-02-10
> 01:22:48.000000000 -0600
> +++ linux-2.6.15.4.patched/drivers/block/loop.c 2006-03-01
> 09:38:48.000000000 -0600

(The patch is wordwrapped)