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

[Anton Altaparmakov <aia21@cam.ac.uk>]

On Wed, 1 Mar 2006, Andrew Morton wrote:
> Robert S Peterson <rpeterso@redhat.com> wrote:
> > 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?

It is simply that it is faster on all file systems that can use 
prepare_write/commit_write and the "performance hungry" would have 
complained if it were to be removed.

> > 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).

*blink* Why should it not work?

cryptoloop is exactly the reason for the performance hit with using 
->write rather than prepare_write/commit_write as we need to allocate a 
temporary page, do the transformation from the source page to the 
temporary page, then send the temporary page to ->write, and finally 
release the page.  Ugly but the only way to allow transformations 
in-flight I could see when writing the ->write using code for loop.c...

Best regards,

	Anton
-- 
Anton Altaparmakov <aia21 at cam.ac.uk> (replace at with @)
Unix Support, Computing Service, University of Cambridge, CB2 3QH, UK
Linux NTFS maintainer / IRC: #ntfs on irc.freenode.net
WWW: http://linux-ntfs.sf.net/ & http://www-stu.christs.cam.ac.uk/~aia21/