Patches can be fetched from github:
https://github.com/adam900710/linux/tree/subpage
Currently the branch also contains partial RW data support (still some
out-of-sync subpage data page status).
Great thanks to David for his effort reviewing and merging the
preparation patches into misc-next.
Now all previously submitted preparation patches are already in
misc-next.
=== What works ===
Just from the patchset:
- Data read
Both regular and compressed data, with csum check.
- Metadata read
This means, with these patchset, 64K page systems can at least mount
btrfs with 4K sector size.
In the subpage branch
- Metadata read write and balance
Not yet full tested due to data write still has bugs need to be
solved.
But considering that metadata operations from previous iteration
is mostly untouched, metadata read write should be pretty stable.
- Baisc data balance
This is new.
- Data read write
Only uncompressed data writes. Fsstress can survive.
But still very rare data csum error, which only happens in bookend data
extents (part of the data extent which is not referred by any file).
This looks like something related to reflink/invalidate page and cow
fixup.
Still invetigating.
=== Needs feedback ===
The following design needs extra comments:
- u16 bitmap
As David mentioned, using u16 as bit map is not the fastest way.
That's also why current bitmap code requires unsigned long (u32) as
minimal unit.
But using bitmap directly would double the memory usage.
Thus the best way is to pack two u16 bitmap into one u32 bitmap, but
that still needs extra investigation to find better practice.
Anyway the skeleton should be pretty simple to expand.
- Separate handling for subpage metadata
Currently the metadata read and (later write path) handles subpage
metadata differently. Mostly due to the page locking must be skipped
for subpage metadata.
I tried several times to use as many common code as possible, but
every time I ended up reverting back to current code.
Thankfully, for data handling we will use the same common code.
- Incompatible subpage strcuture against iomap_page
In btrfs we need extra bits than iomap_page.
This is due to we need sector perfect write for data balance.
E.g. if only one 4K sector is dirty in a 64K page, we should only
write that dirty 4K back to disk, not the full 64K page.
As data balance requires the new data extents to have exactly the
same size as the original ones.
This means, unless iomap_page get extra bits like what we're doing in
btrfs for dirty, we can't merge the btrfs_subpage with iomap_page.
=== Patchset structure ===
Patch 01~03: Existing preparation patches.
Mostly readability related patches found during RW
development
Patch 04~05: New preparation patches.
Mostly related to __process_pages_contig().
Patch 06~10: Subpage handling for extent buffer allocation and
freeing
Patch 11~22: Subpage handling for extent buffer read path
=== Changelog ===
v1:
- Separate the main implementation from previous huge patchset
Huge patchset doesn't make much sense.
- Use bitmap implementation
Now page::private will be a pointer to btrfs_subpage structure, which
contains bitmaps for various page status.
v2:
- Use page::private as btrfs_subpage for extra info
This replace old extent io tree based solution, which reduces latency
and don't require memory allocation for its operations.
- Cherry-pick new preparation patches from RW development
Those new preparation patches improves the readability by their own.
v3:
- Make dummy extent buffer to follow the same subpage accessors
Fsstress exposed several ASSERT() for dummy extent buffers.
It turns out we need to make dummy extent buffer to own the same
btrfs_subpage structure to make eb accessors to work properly
- Two new small __process_pages_contig() related preparation patches
One to make __process_pages_contig() to enhance the error handling
path for locked_page, one to merge one macro.
- Extent buffers refs count update
Except try_release_extent_buffer(), all other eb uses will try to
increase the ref count of the eb.
For try_release_extent_buffer(), the eb refs check will happen inside
the rcu critical section to avoid eb being freed.
- Comment updates
Addressing the comments from the mail list.
Qu Wenruo (22):
btrfs: extent_io: rename @offset parameter to @disk_bytenr for
submit_extent_page()
btrfs: extent_io: refactor __extent_writepage_io() to improve
readability
btrfs: file: update comment for btrfs_dirty_pages()
btrfs: extent_io: update locked page dirty/writeback/error bits in
__process_pages_contig()
btrfs: extent_io: merge PAGE_CLEAR_DIRTY and PAGE_SET_WRITEBACK into
PAGE_START_WRITEBACK
btrfs: extent_io: introduce a helper to grab an existing extent buffer
from a page
btrfs: extent_io: introduce the skeleton of btrfs_subpage structure
btrfs: extent_io: make attach_extent_buffer_page() to handle subpage
case
btrfs: extent_io: make grab_extent_buffer_from_page() to handle
subpage case
btrfs: extent_io: support subpage for extent buffer page release
btrfs: extent_io: attach private to dummy extent buffer pages
btrfs: subpage: introduce helper for subpage uptodate status
btrfs: subpage: introduce helper for subpage error status
btrfs: extent_io: make set/clear_extent_buffer_uptodate() to support
subpage size
btrfs: extent_io: make btrfs_clone_extent_buffer() to be subpage
compatible
btrfs: extent_io: implement try_release_extent_buffer() for subpage
metadata support
btrfs: extent_io: introduce read_extent_buffer_subpage()
btrfs: extent_io: make endio_readpage_update_page_status() to handle
subpage case
btrfs: disk-io: introduce subpage metadata validation check
btrfs: introduce btrfs_subpage for data inodes
btrfs: integrate page status update for read path into
begin/end_page_read()
btrfs: allow RO mount of 4K sector size fs on 64K page system
fs/btrfs/Makefile | 3 +-
fs/btrfs/compression.c | 10 +-
fs/btrfs/disk-io.c | 82 +++++-
fs/btrfs/extent_io.c | 534 ++++++++++++++++++++++++++++--------
fs/btrfs/extent_io.h | 15 +-
fs/btrfs/file.c | 35 +--
fs/btrfs/free-space-cache.c | 15 +-
fs/btrfs/inode.c | 40 ++-
fs/btrfs/ioctl.c | 5 +-
fs/btrfs/reflink.c | 5 +-
fs/btrfs/relocation.c | 12 +-
fs/btrfs/subpage.c | 39 +++
fs/btrfs/subpage.h | 256 +++++++++++++++++
fs/btrfs/super.c | 7 +
14 files changed, 876 insertions(+), 182 deletions(-)
create mode 100644 fs/btrfs/subpage.c
create mode 100644 fs/btrfs/subpage.h
--
2.29.2
