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

[David Sterba <dsterba@suse.cz>]

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?

[...]
> [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.