Re: [PATCH][RFC] fast file mapping for loop

[Jens Axboe]

 On Mon, Jan 14 2008, Jens Axboe wrote:
> On Mon, Jan 14 2008, Chris Mason wrote:
> > Hello everyone,
> > 
> > Here is a modified version of Jens' patch.  The basic idea is to push
> > the mapping maintenance out of loop and down into the filesystem (ext2
> > in this case).
> > 
> > Two new address_space operations are added, one to map
> > extents and the other to provide call backs into the FS as io is
> > completed.
> > 
> > Still TODO for this patch:
> > 
> > * Add exclusion between filling holes and readers.  This is partly
> > implemented, when a hole is filled by the FS, the extent is flagged as
> > having a hole.  The idea is to check this flag before trying to read
> > the blocks and just send back all zeros.
> > 
> > The flag would be cleared when the blocks filling the hole have been
> > written.
> > 
> > * Exclude page cache readers and writers
> > 
> > * Add a way for the FS to request a commit before telling the higher
> > layers the IO is complete.  This way we can make sure metadata related
> > to holes is on disk before claiming the IO is really done.  COW based
> > filesystems will also needed it.
> > 
> > * Change loop to use fast mapping only when the new address_space op is
> > provided (whoops, forgot about this until just now)
> > 
> > * A few other bits for COW, not really relevant until there
> > is...something COW using it.
> 
> Looks pretty good. Essentially the loop side is 100% the same, it just
> offloads the extent ownership to the fs (where it belongs). So I like
> it. Attaching a small cleanup/fixup patch for loop, don't think it needs
> further explanations.
> 
> One suggestion - free_extent_map(), I would call that put_extent_map()
> instead.

I split and merged the patch into five bits (added ext3 support), so
perhaps that would be easier for people to read/review. Attached and
also exist in the loop-extent_map branch here:

http://git.kernel.dk/?p=linux-2.6-block.git;a=shortlog;h=loop-extent_map

-- 
Jens Axboe