Re: [PATCH v5 00/18] btrfs: add read-only support for subpage sector size

[Josef Bacik <josef@toxicpanda.com>]

On 1/26/21 3:33 AM, Qu Wenruo wrote:
> Patches can be fetched from github:
> https://github.com/adam900710/linux/tree/subpage
> Currently the branch also contains partial RW data support (still some
> ordered extent and data csum mismatch problems)
> 
> Great thanks to David/Nikolay/Josef for their effort reviewing and
> merging the preparation patches into 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 read-only.
> This should provide the ability to migrate data at least.
> 
> While on the github branch, there are already experimental RW supports,
> there are still ordered extent related bugs for me to fix.
> Thus only the RO part is sent for review and testing.
> 
> === Patchset structure ===
> Patch 01~02:	Preparation patches which don't have functional change
> Patch 03~12:	Subpage metadata allocation and freeing
> Patch 13~15:	Subpage metadata read path
> Patch 16~17:	Subpage data read path
> Patch 18:	Enable subpage RO support
> 
> === 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.
> 
> v4:
> - Get rid of btrfs_subpage::tree_block_bitmap
>    This is to reduce lock complexity (no need to bother extra subpage
>    lock for metadata, all locks are existing locks)
>    Now eb looking up mostly depends on radix tree, with small help from
>    btrfs_subpage::under_alloc.
>    Now I haven't experieneced metadata related problems any more during
>    my local fsstress tests.
> 
> - Fix a race where metadata page dirty bit can race
>    Fixed in the metadata RW patchset though.
> 
> - Rebased to latest misc-next branch
>    With 4 patches removed, as they are already in misc-next.
> 
> v5:
> - Use the updated version from David as base
>    Most comment/commit message update should be kept as is.
> 
> - A new separate patch to move UNMAPPED bit set timing
> 
> - New comment on why we need to prealloc subpage inside a loop
>    Mostly for further 16K page size support, where we can have
>    eb across multiple pages.
> 
> - Remove one patch which is too RW specific
>    Since it introduces functional change which only makes sense for RW
>    support, it's not a good idea to include it in RO support.
> 
> - Error handling fixes
>    Great thanks to Josef.
> 
> - Refactor btrfs_subpage allocation/freeing
>    Now we have btrfs_alloc_subpage() and btrfs_free_subpage() helpers to
>    do all the allocation/freeing.
>    It's pretty easy to convert to kmem_cache using above helpers.
>    (already internally tested using kmem_cache without problem, in fact
>     it's all the problems found in kmem_cache test leads to the new
>     interface)
> 
> - Use btrfs_subpage::eb_refs to replace old under_alloc
>    This makes checking whether the page has any eb left much easier.
> 
> Qu Wenruo (18):
>    btrfs: merge PAGE_CLEAR_DIRTY and PAGE_SET_WRITEBACK to
>      PAGE_START_WRITEBACK
>    btrfs: set UNMAPPED bit early in btrfs_clone_extent_buffer() for
>      subpage support
>    btrfs: introduce the skeleton of btrfs_subpage structure
>    btrfs: make attach_extent_buffer_page() handle subpage case
>    btrfs: make grab_extent_buffer_from_page() handle subpage case
>    btrfs: support subpage for extent buffer page release

I don't have this patch in my inbox so I can't reply to it directly, but you 
include refcount.h, but then use normal atomics.  Please used the actual 
refcount_t, as it gets us all the debugging stuff that makes finding problems 
much easier.  Thanks,

Josef