[PATCH v2 00/26] btrfs: limited subpage compressed write support

Qu Wenruo
2021-08-29T05:25:04Z

The patchset can be fetched from github:
https://github.com/adam900710/linux/tree/compression

The branch is based on the previously submitted subpage enablement
patchset.
The target merge window is v5.16 or v5.17.

=== What's working ===

Delalloc range which is fully page aligned can be compressed with
64K page size and 4K sector size (AKA, subpage).

With current patchset, it can pass most "compress" test group, except
btrfs/106, whose golden output is bound to 4K page size, thus test case
needs to be updated.

And as a basic requirement, 4K page size systems still pass the regular
fstests runs.

=== What's not working ===

Delalloc range not fully page aligned will not go through compression.

That's to say, the following inode will go through different write path:

0 32K 64K 96K 128K
|///////////////| |///////|
| \- Will not be compressed
|
\- Will be compressed

This will reduce the chance of compression obviously.

But all involved patches will be the basis for later sector perfect
compression support.

The limitation is mostly introduced by two factors:

- How we handle the locked page of a async cow delalloc range
Currently we unlock the first page unconditionally.
Even with the patchset, we still follows the behavior.

This means we can't have two async cow range shares the same
page.
This can be enhanced to use subpage::writers, but the next
problem will prevent us doing so.

- No way to ensure an async cow range not to unlock the page while
we still have delalloc range in the page

This is caused by how we run delalloc range in a page.
For regular sectorsize, it's not a problem as we have at most one
sector for a page.

But for subpage case, we can have multiple sectors in one page.
If we submit an async cow, it may try to unlock the page while
we are still running the next delalloc range of the page.

The correct way here is to find and lock all delalloc range inside a
page, update the subpage::writers properly, then run each delalloc
range, so that the page won't be unlocked half way.

=== Patch structure ===

Patch 01~04: Small and safe cleanups
Patch 05: Make compressed readahead to be subpage compatble
Patch 06~14: Optimize compressed read/write path to determine stripe
boundary in a per-bio base
Patch 15~16: Extra code refactor/cleanup for compressed path

Patch 17~25: Make compressed write path to be subpage compatible
Patch 26: Enable limited subpage compressed write support

Patch 01~16 may be a good candidate for early merge, as real heavy
lifting part starts at patch 17.

While patch 01~04 are really small and safe cleanups, which can be
merged even earlier than subpage enablement patchset.

While the patches 06~14 is quite some refactor, it may be needed for the
read-only support for compression.
As the read-time bio split is also a critical part for read-only
compressed data support.

I don't have any good idea to push those read path fixes to v5.15
branches for now.
Maybe I need to craft a hot-fix for read-write support.

Qu Wenruo (26):
btrfs: remove unused parameter @nr_pages in add_ra_bio_pages()
btrfs: remove unnecessary parameter @delalloc_start for
writepage_delalloc()
btrfs: use async_chunk::async_cow to replace the confusing pending
pointer
btrfs: don't pass compressed pages to
btrfs_writepage_endio_finish_ordered()
btrfs: make add_ra_bio_pages() to be subpage compatible
btrfs: introduce compressed_bio::pending_sectors to trace compressed
bio more elegantly
btrfs: add subpage checked bitmap to make PageChecked flag to be
subpage compatible
btrfs: handle errors properly inside btrfs_submit_compressed_read()
btrfs: handle errors properly inside btrfs_submit_compressed_write()
btrfs: introduce submit_compressed_bio() for compression
btrfs: introduce alloc_compressed_bio() for compression
btrfs: make btrfs_submit_compressed_read() to determine stripe
boundary at bio allocation time
btrfs: make btrfs_submit_compressed_write() to determine stripe
boundary at bio allocation time
btrfs: remove unused function btrfs_bio_fits_in_stripe()
btrfs: refactor submit_compressed_extents()
btrfs: cleanup for extent_write_locked_range()
btrfs: make compress_file_range() to be subpage compatible
btrfs: make btrfs_submit_compressed_write() to be subpage compatible
btrfs: make end_compressed_bio_writeback() to be subpage compatble
btrfs: make extent_write_locked_range() to be subpage compatible
btrfs: extract uncompressed async extent submission code into a new
helper
btrfs: rework lzo_compress_pages() to make it subpage compatible
btrfs: teach __extent_writepage() to handle locked page differently
btrfs: allow page to be unlocked by btrfs_page_end_writer_lock() even
if it's locked by plain page_lock()
btrfs: don't run delalloc range which is beyond the locked_page to
prevent deadlock for subpage compression
btrfs: only allow subpage compression if the range is fully page
aligned

fs/btrfs/compression.c | 676 ++++++++++++++++++-------------
fs/btrfs/compression.h | 4 +-
fs/btrfs/ctree.h | 2 -
fs/btrfs/extent_io.c | 123 ++++--
fs/btrfs/extent_io.h | 3 +-
fs/btrfs/file.c | 20 +-
fs/btrfs/free-space-cache.c | 6 +-
fs/btrfs/inode.c | 452 +++++++++++----------
fs/btrfs/lzo.c | 280 ++++++-------
fs/btrfs/reflink.c | 2 +-
fs/btrfs/subpage.c | 102 ++++-
fs/btrfs/subpage.h | 4 +
fs/btrfs/tests/extent-io-tests.c | 12 +-
13 files changed, 1002 insertions(+), 684 deletions(-)

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2.32.0