Re: [PATCH] btrfs: reject zoned RW mount if sectorsize is smaller than page size

[Qu Wenruo <wqu@suse.com>]

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On 2024/2/12 21:44, David Sterba wrote:
> On Mon, Feb 12, 2024 at 03:46:15PM +1030, Qu Wenruo wrote:
>> [BUG]
>> There is a bug report that with zoned device and sectorsize is smaller
>> than page size (aka, subpage), btrfs would crash with a very basic
>> workload:
>>
>>   # getconfig PAGESIZE
>>   16384
>>   # mkfs.btrfs -f $dev -s 4k
>>   # mount $dev $mnt
>>   # $fsstress -w -n 8 -s 1707820327 -v -d $mnt
>>   # umount $mnt
>>
>> The crash would look like this (with CONFIG_BTRFS_ASSERT enabled):
>>
>>   assertion failed: block_start != EXTENT_MAP_HOLE, in fs/btrfs/extent_io.c:1384
> 
> This is the same as what Josef fixed in
> https://github.com/btrfs/linux/commit/400bb013912dac637c7d6826407be580ea8ef9cc
> "btrfs: don't drop extent_map for free space inode on write error"
> but not on zoned+subpage, is it really a different error you're fixing?

Yes, it's really a different bug.

Although the root cause is a little more complex, it has to be explained 
through the whole CAUSE section.

> 
> [...]
>> [WORKAROUND]
>> A proper fix requires some big changes to delalloc workload, to allow
>> extent_write_locked_range() to handle multiple different entries with
>> the same @locked_page.
>>
>> So for now, disable read-write support for subpage zoned btrfs.
>>
>> The problem can only be solved if subpage btrfs can handle subpage
>> compression, which need quite some work on the delalloc procedure for
>> the @locked_page handling.
> 
> Ok, this on itself is a valid reason to disable the support.

But this reminds me that, I have to work on the proper delalloc support 
for subpage.

My current plan is:

1. Introduce subpage LOCK bitmap
    Not just only an atomic counter.

2. Make find_lock_delalloc_range() to populate that lock bitmap

3. Make find_lock_delalloc_range() to find all delalloc range of
    a page and properly lock them
    This would not change the behavior for page sized sectorsize.

4. Make find_lock_delalloc_range() to skip the page if it's already
    being locked before.
    This is also a subpage specific behavior

With above planned work, btrfs should be able to handle subpage unlock, 
so that we won't need weird @locked_page handling.

Hopefully this would solve both the subpage+zoned and full sector 
aligned subpage compression support.

Thanks,
Qu

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