[PATCH v2 00/19] btrfs: add read-only support for subpage sector size

Qu Wenruo
2020-09-15T05:35:41Z

Patches can be fetched from github:
https://github.com/adam900710/linux/tree/subpage

Currently btrfs only allows to mount fs with sectorsize == PAGE_SIZE.

That means, for 64K page size system, they can only use 64K sector size
fs.
This brings a big compatible problem for btrfs.

This patch is going to slightly solve the problem by, allowing 64K
system to mount 4K sectorsize fs in read-only mode.

The main objective here, is to remove the blockage in the code base, and
pave the road to full RW mount support.

== What works ==

Existing regular page sized sector size support
Subpage read-only Mount (with all self tests and ASSERT)
Subpage metadata read (including all trees and inline extents, and csum checking)
Subpage uncompressed data read (with csum checking)

== What doesn't work ==

Read-write mount (see the subject)
Compressed data read

== Challenge we meet ==

The main problem is metadata, where we have several limitations:
- We always read the full page of a metadata
In subpage case, one full page can contain several tree blocks.

- We use page::private to point to extent buffer
This means we currently can only support one-page-to-one-extent-buffer
mapping.
For subpage size support, we need one-page-to-multiple-extent-buffer
mapping.

== Solutions ==

So here for the metadata part, we use the following methods to
workaround the problem:

- Completely rely on extent_io_tree for metadata status/locking
Now for subpage metadata, page::private is never utilized. It always
points to NULL.
And we only utilize private page status, other status
(locked/uptodate/dirty/...) are all ignored.

Instead, page lock is replayed by EXTENT_LOCK of extent_io_tree.
Page uptodate is replaced by EXTENT_UPTODATE of extent_io_tree.
And if a range has extent buffer is represented by EXTENT_NEW.

This provides the full potential for later RW support.

- Do subpage read for metadata
Now we do proper subpage read for both data and metadata.
For metadata we never merge bio for adjacent tree blocks, but always
submit one bio for one tree block.
This allows us to do proper verification for each tree blocks.

For data part, it's pretty simple, all existing infrastructure can be
easily converted to support subpage read, without any subpage specific
handing yet.

== Patchset structure ==

The structure of the patchset:
Patch 01~15: Preparation patches for data and metadata subpage read support.
These patches can be merged without problem, and work for
both regular and subpage case.
This part can conflict with Nikolay's latest cleanup, but
the conflicts should be pretty controllable.

Patch 16~19: Patches for metadata subpage read support.
The main part of the patchset. It converts metadata to
purely extent_io_tree based solution for subpage read.

In theory, page sized routine can also be converted to
extent_io_tree. But that would be another topic in the
future.

The number of patches is the main reason I'm submitting them to the mail
list. As there are too many preparation patches already.

Qu Wenruo (19):
btrfs: extent-io-tests: remove invalid tests
btrfs: remove the unnecessary parameter @start and @len for
check_data_csum()
btrfs: calculate inline extent buffer page size based on page size
btrfs: remove the open-code to read disk-key
btrfs: make btrfs_fs_info::buffer_radix to take sector size devided
values
btrfs: don't allow tree block to cross page boundary for subpage
support
btrfs: update num_extent_pages() to support subpage sized extent
buffer
btrfs: handle sectorsize < PAGE_SIZE case for extent buffer accessors
btrfs: make csum_tree_block() handle sectorsize smaller than page size
btrfs: add assert_spin_locked() for attach_extent_buffer_page()
btrfs: extract the extent buffer verification from
btree_readpage_end_io_hook()
btrfs: extent_io: only require sector size alignment for page read
btrfs: make btrfs_readpage_end_io_hook() follow sector size
btrfs: make btree inode io_tree has its special owner
btrfs: don't set extent_io_tree bits for btree inode at endio time
btrfs: use extent_io_tree to handle subpage extent buffer allocation
btrfs: implement subpage metadata read and its endio function
btrfs: implement btree_readpage() and try_release_extent_buffer() for
subpage
btrfs: allow RO mount of 4K sector size fs on 64K page system

fs/btrfs/btrfs_inode.h | 12 +
fs/btrfs/ctree.c | 13 +-
fs/btrfs/ctree.h | 38 +++-
fs/btrfs/disk-io.c | 217 ++++++++++++++----
fs/btrfs/extent-io-tree.h | 8 +
fs/btrfs/extent_io.c | 376 +++++++++++++++++++++++++++----
fs/btrfs/extent_io.h | 19 +-
fs/btrfs/inode.c | 40 +++-
fs/btrfs/ordered-data.c | 8 +
fs/btrfs/qgroup.c | 4 +
fs/btrfs/struct-funcs.c | 18 +-
fs/btrfs/super.c | 7 +
fs/btrfs/tests/extent-io-tests.c | 26 +--
13 files changed, 642 insertions(+), 144 deletions(-)

--
2.28.0