Re: [PATCH 00/17] btrfs: add read-only support for subpage sector size

[Qu Wenruo <wqu@suse.com>]

On 2020/9/8 下午3:52, Qu Wenruo wrote:
> 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:

This is pretty different from what Chanda submitted several years ago.

Chanda chooses to base its work on Josef's attempt to kill btree_inode
and use kmalloc memory for tree blocks.
That idea is in fact pretty awesome, it solves a lot of problem and
makes btree read/write way easier.

The problem is, that attempt to kill btree_inode is exposing a big
behavior change, which brings a lot of uncertainty to the following
development.

While this patchset choose to use the existing btree_inode mechanism to
make it easier to be merged.

Personally speaking, I still like the idea of btree_inode kill.
If we could get an agreement on the direction we take, it would be much
better for the future.

Thanks,
Qu
> 
> - Introduce subpage_eb_mapping structure to do bitmap
>   Now for subpage, page::private points to a subpage_eb_mapping
>   structure, which has a bitmap to mapping one page to multiple extent
>   buffers.
> 
> - Still do full page read for metadata
>   This means, at read time, we're not reading just one extent buffer,
>   but possibly many more.
>   In that case, we first do tree block verification for the tree blocks
>   triggering the read, and mark the page uptodate.
> 
>   For newly incoming tree block read, they will check if the tree block
>   is verified. If not verified, even if the page is uptodate, we still
>   need to check the extent buffer.
> 
>   By this all subpage extent buffers are verified properly.
> 
> 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 01~11: Preparation patches for metadata subpage read support.
>              These patches can be merged without problem, and work for
>              both regular and subpage case.
> Patch 12~14: Patches for data subpage read support.
>              These patches works for both cases.
> 
> That means, patch 01~14 can be applied to current kernel, and shouldn't
> affect any existing behavior.
> 
> Patch 15~17: Subpage metadata read specific patches.
>              These patches introduces the main part of the subpage
>              metadata read support.
> 
> 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 (17):
>   btrfs: extent-io-tests: remove invalid tests
>   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: handle sectorsize < PAGE_SIZE case for extent buffer accessors
>   btrfs: make csum_tree_block() handle sectorsize smaller than page size
>   btrfs: refactor how we extract extent buffer from page for
>     alloc_extent_buffer()
>   btrfs: refactor btrfs_release_extent_buffer_pages()
>   btrfs: add assert_spin_locked() for attach_extent_buffer_page()
>   btrfs: extract the extent buffer verification from
>     btree_readpage_end_io_hook()
>   btrfs: remove the unnecessary parameter @start and @len for
>     check_data_csum()
>   btrfs: extent_io: only require sector size alignment for page read
>   btrfs: make btrfs_readpage_end_io_hook() follow sector size
>   btrfs: introduce subpage_eb_mapping for extent buffers
>   btrfs: handle extent buffer verification proper for subpage size
>   btrfs: allow RO mount of 4K sector size fs on 64K page system
> 
>  fs/btrfs/ctree.c                 |  13 +-
>  fs/btrfs/ctree.h                 |  38 ++-
>  fs/btrfs/disk-io.c               | 111 ++++---
>  fs/btrfs/disk-io.h               |   1 +
>  fs/btrfs/extent_io.c             | 538 +++++++++++++++++++++++++------
>  fs/btrfs/extent_io.h             |  19 +-
>  fs/btrfs/inode.c                 |  40 ++-
>  fs/btrfs/struct-funcs.c          |  18 +-
>  fs/btrfs/super.c                 |   7 +
>  fs/btrfs/tests/extent-io-tests.c |  26 +-
>  10 files changed, 633 insertions(+), 178 deletions(-)
>