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
>  ------------[ cut here ]------------
>  kernel BUG at fs/btrfs/extent_io.c:1384!
>  CPU: 0 PID: 872 Comm: kworker/u9:2 Tainted: G           OE      6.8.0-rc3-custom+ #7
>  Hardware name: QEMU KVM Virtual Machine, BIOS edk2-20231122-12.fc39 11/22/2023
>  Workqueue: writeback wb_workfn (flush-btrfs-8)
>  pc : __extent_writepage_io+0x404/0x460 [btrfs]
>  lr : __extent_writepage_io+0x404/0x460 [btrfs]
>  Call trace:
>   __extent_writepage_io+0x404/0x460 [btrfs]
>   extent_write_locked_range+0x16c/0x460 [btrfs]
>   run_delalloc_cow+0x88/0x118 [btrfs]
>   btrfs_run_delalloc_range+0x128/0x228 [btrfs]
>   writepage_delalloc+0xb8/0x178 [btrfs]
>   __extent_writepage+0xc8/0x3a0 [btrfs]
>   extent_write_cache_pages+0x1cc/0x460 [btrfs]
>   extent_writepages+0x8c/0x120 [btrfs]
>   btrfs_writepages+0x18/0x30 [btrfs]
>   do_writepages+0x94/0x1f8
>   __writeback_single_inode+0x4c/0x388
>   writeback_sb_inodes+0x208/0x4b0
>   wb_writeback+0x118/0x3c0
>   wb_do_writeback+0xbc/0x388
>   wb_workfn+0x80/0x240
>   process_one_work+0x154/0x3c8
>   worker_thread+0x2bc/0x3e0
>   kthread+0xf4/0x108
>   ret_from_fork+0x10/0x20
>  Code: 9102c021 90000be0 91378000 9402bf53 (d4210000)
>  ---[ end trace 0000000000000000 ]---
> 
> [CAUSE]
> There are several factors causing the problem:
> 
> 1. __extent_writepage_io() requires all dirty ranges to have delalloc
>    executed
>    This can be solved by adding @start and @len parameter to only submit
>    IO for a subset of the page, and update several involved helpers to
>    do subpage checks.
> 
>    So this is not a big deal.
> 
> 2. Subpage only accepts for full page aligned ranges for
>    extent_write_locked_range()
>    For zoned device, regular COW is switched to utilize
>    extent_write_locked_range() to submit the IO.
> 
>    But the caller, run_delalloc_cow() can be called to run on a subpage
>    range, e.g.
> 
>    0     4K     8K    12K     16K
>    |/////|      |/////|
> 
>    Where |///| is the dirtied range.
> 
>    In that case, btrfs_run_delalloc_range() would call run_delalloc_cow(),
>    which would call extent_write_locked_range() for [0, 4K), and unlock
>    the whole [0, 16K) page.
> 
>    But btrfs_run_delalloc_range() would again be called for range [8K,
>    12K), as there are still dirty range left.
>    In that case, since the whole page is already unlocked by previous
>    iteration, and would cause different ASSERT()s inside
>    extent_write_locked_range().
> 
>    That's also why compression for subpage cases require fully page
>    aligned range.
> 
> [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.
> 
> Reported-by: HAN Yuwei <hrx@bupt.moe>
> Link: https://lore.kernel.org/all/1ACD2E3643008A17+da260584-2c7f-432a-9e22-9d390aae84cc@bupt.moe/
> CC: stable@vger.kernel.org # 5.10+
> Signed-off-by: Qu Wenruo <wqu@suse.com>
> ---
>  fs/btrfs/disk-io.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
> index c3ab268533ca..85cd23aebdd6 100644
> --- a/fs/btrfs/disk-io.c
> +++ b/fs/btrfs/disk-io.c
> @@ -3193,7 +3193,8 @@ int btrfs_check_features(struct btrfs_fs_info *fs_info, bool is_rw_mount)
>  	 * part of @locked_page.
>  	 * That's also why compression for subpage only work for page aligned ranges.
>  	 */
> -	if (fs_info->sectorsize < PAGE_SIZE && btrfs_is_zoned(fs_info) && is_rw_mount) {
> +	if (fs_info->sectorsize < PAGE_SIZE &&
> +	    btrfs_fs_incompat(fs_info, ZONED) && is_rw_mount) {
>  		btrfs_warn(fs_info,
>  	"no zoned read-write support for page size %lu with sectorsize %u",
>  			   PAGE_SIZE, fs_info->sectorsize);

This does not match any code I see in usual branches and it also does
not apply so it can't be picked as a fix.