Re: [RFC] new ->perform_write fop

[Dave Chinner]

On Thu, May 13, 2010 at 11:30:57PM -0400, Josef Bacik wrote:
> 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)

Actually, i was thinking that the RESERVE call determines the size
of the chunk (in the order of 1-4MB maximum). IOWs, we pass in the
start offset of the write, the entire length remaining, and the
RESERVE call determines how much it will allow in one loop.

	written = 0;
	while (bytes_remaining > 0) {
		chunklen = ->allocate(off, bytes_remaining, RESERVE);
		write_begin(&pages, off, chunklen);
		copied = copy_pages(&pages, iov_iter, chunklen);
		.....
		bytes_remaining -= copied;
		off += copied;
		written += copied;
	}


> 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.

Close - except that I'd like like to move the block allocation out of
->write_begin until after the copy into the pages has completed.
Hence write_begin only deals with populating the page cache and
anything filesystem specific like locking or allocating/populating
per-page structures like bufferheads.  i.e. ->write_begin does not
do anything that cannot be undone easily if the copy fails.

> 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.

Well, the "RESERVE" callout I proposed should mean that the
allocation that follows the copy should never fail with ENOSPC. i.e.
if there isn't space, the RESERVE call will fail with ENOSPC, not
the actual ALLOCATE call that occurs after the data is copied.

Basically I was thinking that in the XFS case, RESERVE would scan the
range for holes, and reserve all blocks needed to fill all the holes
in the range. Then the ALLOCATE call would mark them all delalloc.
The space reservation and delalloc is done in a single call right
now, but splitting them is not hard...

> 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.

fault_in_readable() only faults the first page of the first iov
passed in. It can still fail on any other page the iov points to.
And even then, fault_in_readable() does not pin the page it faulted
in memory, so even that can get recycled before we start the copy
and we'd still get EFAULT.

> 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?

Yeah, that's kind of what I was thinking of. After a bit more
thought, if the allocation only covers part of the range (for
whatever reason), we can still allow that allocated part of the
write to complete succesfully, and the UNRESERVE call simply returns
the unused part of the space reservation. This would be a partial
write and be much simpler to deal with than punching out the
allocations that succeeded.

> 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,

buffer_new() would suffice, I think.

In __block_prepare_write(), when a new buffer is mapped that flag is
cleared. If the buffer is not mapped, we call get_blocks, and any
buffer it allocates will be marked buffer_new() again. This is used
to trigger special things in __block_prepare_write() and the flag is
not cleared until write_end (in __block_commit_write). Hence a
buffer with the new flag set after allocation is a buffer that had
space allocated...

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx
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