Btrfs: subpagesize-blocksize: Get rid of whole page writes.

This commit brings back functions that set/clear EXTENT_WRITEBACK bits. These
are required to reliably clear PG_writeback page flag.

Signed-off-by: Chandan Rajendra <chandan@linux.vnet.ibm.com>
Btrfs: subpagesize-blocksize: __btrfs_buffered_write: Reserve/release extents aligned to block size.

Currently, the code reserves/releases extents in multiples of PAGE_CACHE_SIZE
units. Fix this.

Signed-off-by: Chandan Rajendra <chandan@linux.vnet.ibm.com>
Btrfs: subpagesize-blocksize: Define extent_buffer_head.

In order to handle multiple extent buffers per page, first we need to create a
way to handle all the extent buffers that are attached to a page.

This patch creates a new data structure 'struct extent_buffer_head', and moves
fields that are common to all extent buffers in a page from 'struct extent
buffer' to 'struct extent_buffer_head'

Also, this patch moves EXTENT_BUFFER_TREE_REF, EXTENT_BUFFER_DUMMY and
EXTENT_BUFFER_IN_TREE flags from extent_buffer->ebflags  to
extent_buffer_head->bflags.

Signed-off-by: Chandra Seetharaman <sekharan@us.ibm.com>
Signed-off-by: Chandan Rajendra <chandan@linux.vnet.ibm.com>
Btrfs: subpagesize-blocksize: Read tree blocks whose size is <PAGE_CACHE_SIZE.

In the case of subpagesize-blocksize, this patch makes it possible to read
only a single metadata block from the disk instead of all the metadata blocks
that map into a page.

Signed-off-by: Chandan Rajendra <chandan@linux.vnet.ibm.com>
Btrfs: subpagesize-blocksize: Write only dirty extent buffers belonging to a page

For the subpagesize-blocksize scenario, This patch adds the ability to write a
single extent buffer to the disk.

Signed-off-by: Chandan Rajendra <chandan@linux.vnet.ibm.com>
Btrfs: subpagesize-blocksize: Allow mounting filesystems where sectorsize != PAGE_SIZE

This patch allows mounting filesystems with blocksize smaller than the
PAGE_SIZE.

Signed-off-by: Chandra Seetharaman <sekharan@us.ibm.com>
Signed-off-by: Chandan Rajendra <chandan@linux.vnet.ibm.com>
Btrfs: subpagesize-blocksize: Compute and look up csums based on sectorsized blocks.

Checksums are applicable to sectorsize units. The current code uses
bio->bv_len units to compute and look up checksums. This works on machines
where sectorsize == PAGE_CACHE_SIZE. This patch makes the checksum
computation and look up code to work with sectorsize units.

Signed-off-by: Chandan Rajendra <chandan@linux.vnet.ibm.com>
Btrfs: subpagesize-blocksize: __extent_writepage: Write only dirty blocks of a page.

The code now loops across 'ordered extents' instead of 'extent maps' to figure
out the dirty blocks of the page to be submitted for a write operation.

Signed-off-by: Chandan Rajendra <chandan@linux.vnet.ibm.com>
Btrfs: subpagesize-blocksize: fallocate: Work with sectorsized units.

While at it, this commit changes btrfs_truncate_page() to truncate sectorsized
blocks instead of pages. Hence the function has been renamed to
btrfs_truncate_block().

Signed-off-by: Chandan Rajendra <chandan@linux.vnet.ibm.com>
Btrfs: subpagesize-blocksize: btrfs_page_mkwrite: Reserve space in sectorsized units.

In subpagesize-blocksize scenario, if i_size occurs in a block which is not
the last block in the page, then the space to be reserved should be calculated
appropriately.

Signed-off-by: Chandan Rajendra <chandan@linux.vnet.ibm.com>
Btrfs: subpagesize-blocksize: Search for all ordered extents that could span across a page.

In subpagesize-blocksize scenario it is not sufficient to search using the
first byte of the page to make sure that there are no ordered extents
present across the page. Fix this.

Signed-off-by: Chandan Rajendra <chandan@linux.vnet.ibm.com>