Re: [RFC] new ->perform_write fop

[Dave Chinner]

On Wed, May 12, 2010 at 09:39:27PM -0400, Josef Bacik wrote:
> On Wed, May 12, 2010 at 05:24:04PM -0400, Josef Bacik wrote:
> > Hello,
> > 
> > I just started adding aio_write to Btrfs and I noticed we're duplicating _alot_
> > of the generic stuff in mm/filemap.c, even though the only thing thats really
> > unique is the fact that we copy userspace pages in chunks rather than one page a
> > t a time.  What would be best is instead of doing write_begin/write_end with
> > Btrfs, it would be nice if we could just do our own perform_write instead of
> > generic_perform_write.  This way we can drop all of these generic checks we have
> > that we copied from filemap.c and just got to the business of actually writing
> > the data.  I hate to add another file operation, but it would _greatly_ reduce
> > the amount of duplicate code we have.  If there is no violent objection to this
> > I can put something together quickly for review.  Thanks,
> >
> 
> I just got a suggestion from hpa about instead just moving what BTRFS does into
> the generic_perform_write.  What btrfs does is allocates a chunk of pages to
> cover the entirety of the write, sets everything up, does the copy from user
> into the pages, and tears everything down, so essentially what
> generic_perform_write does, just with more pages.

Except that btrfs does things in a very different manner to most
other filesystems ;)

> I could modify
> generic_perform_write and the write_begin/write_end aops to do this, where
> write_begin will return how many pages it allocated, copy in all of the
> userpages into the pages we allocated at once, and then call write_end with the
> pages we allocated in write begin.  Then I could just make btrfs do
> write_being/write_end.  So which option seems more palatable?  Thanks,

I can see how this would work for btrfs, but the issue is how any
other filesystem would handle it.

I've been trying to get my head around how any filesystem using
bufferheads and generic code can do multipage writes using
write_begin/write_end without modifying the interface, and I just
threw away my second attempt because the error handling just
couldn't be handled cleanly without duplicating the entire
block_write_begin path in each filesystem that wanted to do
multipage writes.

The biggest problem is that block allocation is intermingled with
allocating and attaching bufferheads to pages, hence error handling
can get really nasty and is split across a call chain 3 or 4
functions deep.  The error handling is where I'm finding all the
dangerous and hard-to-kill demons lurking in dark corners. I suspect
there's a grue in there somewhere, too. ;)

Separating the page+bufferhead allocation and block allocation would
make this much simpler but I can't fit that easily into the existing
interfaces.

Hence I think that write_begin/copy pages/write_end is not really
suited to multipage writes when allocation is done in write_begin
and the write can then fail in a later stage without a simple method
of undoing the allocation. We don't have any hole punch interfaces
to the filesystems (and I think only XFS supports that functionality
right now), so handling errors after allocation becomes rather
complex, especially when you have multiple blocks per page. 

Hence I've independently come to the conclusion that delaying the
allocation until *after* the copy as btrfs does is probably the best
approach to take here.  This largely avoids the error handling
complexity because the write operation is an all-or-nothing
operation. btrfs has separate hooks for space reservation and
releasing the reservation and doesn't commit to actually allocating
anything until the copying is complete. Hence cleanup is simple no
matter where a failure occurs.

Personally, I'm tending towards killing the get_blocks() callback as
the first step in this process - turn it into a real inode/address
space operation (say ->allocate) so we can untangle the write path
somewhat (lots of filesystem just provide operations as wrappers to
provide a fs-specific get_blocks callback to generic code.  If the
"create" flag is then converted to a "command" field, the interface
can pass "RESERVE", "ALLOC", "CREATE", etc to allow different
operations to be clearly handled.

e.g.:

	->allocate(mapping, NULL, off, len, RESERVE)
		reserves necessary space for write
	->write_begin
		grab pages into page cache
		attach bufferheads (if required)
	fail -> goto drop pages
	copy data into pages
	fail -> goto drop pages
	->allocate(mapping, pages, off, len, ALLOC)
		allocates reserved space (if required)
		sets up/maps/updates bufferheads/extents
	fail -> goto drop pages
	->write_end
		set pages dirty + uptodate
	done

drop_pages:
	->allocate(mapping, NULL, off, len, UNRESERVE)
	if needed, zero partial pages
	release pages, clears uptodate.

Basically this allows the copying of data before any allocation is
actually done, but also allows ENOSPC to be detected before starting
the copy. The filesystem can call whatver helpers it needs inside
->get_blocks(ALLOC) to set up bufferhead/extent state to match
what has been reserved/allocated/mapped in the RESERVE call.

This will work for btrfs, and it will work for XFS and I think it
will work for other filesystems that are using bufferheads as well.
For those filesystems that will only support a page at a time, then
they can continue to use the current code, but should be able to be
converted to the multipage code by making RESERVE and UNRESERVE
no-ops, and ALLOC does what write_begin+get_blocks currently does to
set up block mappings.

Thoughts?

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx
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