Re: [RFC] new ->perform_write fop

[Josef Bacik]

On Fri, May 14, 2010 at 11:00:42AM +1000, Dave Chinner wrote:
> 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?
>
So this is what I had envisioned, we make write_begin take a nr_pages pointer
and tell it how much data we have to write, then in the filesystem we allocate
as many pages as we feel like, idealy something like

min(number of pages we need for the write, some arbitrary limit for security)

and then we allocate all those pages.  Then you can pass them into
block_write_begin, which will walk the pages, allocating buffer heads and
allocating the space as needed.

Now since we're coming into write_begin with "we want to write X bytes" we can
go ahead and do the enospc checks for X bytes, and then if we are good to go,
chances are we won't fail.

Except if we're overwriting a holey section of the file, we're going to be
screwed in both your way and my way.  My way probably would be the most likely
to fail, since we could fail to do the copy_from_user, but hopefully the segment
checks and doing the fault_in_readable before all of this would keep those
problems to a minimum.

In your case the only failure point is in the allocate step.  If we fail on down
the line after we've done some hole filling, we'll be hard pressed to go back
and free up those blocks.  Is that what you are talking about, having the
allocate(UNRESERVE) thing being able to go back and figure out what should have
been holes needs to be holes again?  If thats the case then I think your idea
will work and is probably the best way to move forward.  But from what I can
remember about how all this works with buffer heads, there's not a way to say
"we _just_ allocated this recently".  Thanks,

Josef
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