[RFC PATCH 0/8] btrfs iomap support

[Goldwyn Rodrigues <rgoldwyn@suse.de>]

This patch series attempts to use kernels iomap for btrfs. Currently,
it covers buffered writes only, but I intend to add some other iomap
uses once this gets through. I am sending this as an RFC because I
would like to find ways to improve the solution since some changes
require adding more functions to the iomap infrastructure which I
would try to avoid. I still have to remove some kinks as well such
as -o compress. I have posted some questions in the individual
patches and would appreciate some input to those.

Some of the problems I faced is:

1. extent locking: While we perform the extent locking for writes,
we need to perform any reads because of non-page-aligned calls before
locking can be done. This requires reading the page, increasing their
pagecount and "letting it go". The iomap infrastructure uses
buffer_heads wheras btrfs uses bio and hence needs to call readpage
exclusively. The "letting it go" part makes me somewhat nervous of
conflicting reads/writes, even though we are protected under i_rwsem.
Is readpage_nolock() a good idea? The extent locking sequence is a
bit weird, with locks and unlock happening in different functions.

2. btrfs pages use PagePrivate to store EXTENT_PAGE_PRIVATE which is not used anywhere.
However, a PagePrivate flag is used for try_to_release_buffers(). Can
we do away with PagePrivate for data pages? The same with PageChecked.
How and why is it used (I guess -o compress)

3. I had to stick information which will be required from iomap_begin()
to iomap_end() in btrfs_iomap which is a pointer in btrfs_inode. Is
there any other place/way we can transmit this information. XFS only
performs allocations and deallocations so it just relies of bmap code
for it.

Suggestions/Criticism welcome.

-- 
Goldwyn