Re: [PATCH] btrfs: zoned: don't submit orphaned extent buffers

在 2026/7/3 15:24, Johannes Thumshirn 写道:
> On a zoned filesystem btree_writepages() can encounter a dirty metadata
> extent buffer whose block group no longer exists. Submitting a write for
> such a buffer maps it to a stale/removed block-group and leaves the folio
> under writeback forever, hanging later in filemap_fdatawait_range(), for
> example the iput(btree_inode) in close_ctree(), which then hangs unmount.
> 
> This is caused by btrfs_clear_buffer_dirty() not clearing the dirty bit of
> a freed tree block but it sets EXTENT_BUFFER_ZONED_ZEROOUT and keeps the
> buffer dirty so that it is still written out to keep the zone's
> meta_write_pointer advancing sequentially. So a freed metadata block
> legitimately stays dirty until that zero-write completes.
> 
> Dropping these buffers is safe: the block group is empty, so they are
> stale, unreferenced, already-freed blocks. Once the zone is reset their
> zero-write is unneeded. Instead of submitting a such a write, finish the
> writeback immediately.
> 
> Reported-by: Shin'ichiro Kawasaki <[email protected]>
> Fixes: 7db94301a980 ("btrfs: zoned: introduce block group context to btrfs_eb_write_context")
> Cc: [email protected]
> Signed-off-by: Johannes Thumshirn <[email protected]>
> ---
>   fs/btrfs/extent_io.c | 16 ++++++++++++++--
>   1 file changed, 14 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
> index 0edd532174fa..4a029ae719e9 100644
> --- a/fs/btrfs/extent_io.c
> +++ b/fs/btrfs/extent_io.c
> @@ -2280,7 +2280,8 @@ static void prepare_eb_write(struct extent_buffer *eb)
>   }
>   
>   static noinline_for_stack void write_one_eb(struct extent_buffer *eb,
> -					    struct writeback_control *wbc)
> +					    struct writeback_control *wbc,
> +					    bool submit)
>   {
>   	struct btrfs_fs_info *fs_info = eb->fs_info;
>   	struct btrfs_bio *bbio;
> @@ -2310,6 +2311,12 @@ static noinline_for_stack void write_one_eb(struct extent_buffer *eb,
>   		wbc_account_cgroup_owner(wbc, folio, range_len);
>   		folio_unlock(folio);
>   	}
> +
> +	if (!submit) {
> +		btrfs_bio_end_io(bbio, BLK_STS_OK);
> +		return;
> +	}
> +
>   	/*
>   	 * If the fs is already in error status, do not submit any writeback
>   	 * but immediately finish it.
> @@ -2397,6 +2404,8 @@ int btree_writepages(struct address_space *mapping, struct writeback_control *wb
>   		struct extent_buffer *eb;
>   
>   		while ((eb = eb_batch_next(&batch)) != NULL) {
> +			bool submit = true;
> +
>   			ctx.eb = eb;
>   
>   			ret = btrfs_check_meta_write_pointer(eb->fs_info, &ctx);
> @@ -2411,6 +2420,9 @@ int btree_writepages(struct address_space *mapping, struct writeback_control *wb
>   				continue;
>   			}
>   
> +			if (btrfs_is_zoned(fs_info) && !ctx.zoned_bg)
> +				submit = false;
> +
>   			if (!lock_extent_buffer_for_io(eb, wbc))
>   				continue;
>   
> @@ -2420,7 +2432,7 @@ int btree_writepages(struct address_space *mapping, struct writeback_control *wb
>   				btrfs_schedule_zone_finish_bg(ctx.zoned_bg, eb);
>   				ctx.zoned_bg->meta_write_pointer += eb->len;
>   			}
> -			write_one_eb(eb, wbc);
> +			write_one_eb(eb, wbc, submit);

I understand this is the minimal fix, but I can't help but wondering, 
would it be more instinctual to release all ebs inside a zoned metadata 
bg when freeing the bg?

Thanks,
Qu

>   		}
>   		nr_to_write_done = (wbc->nr_to_write <= 0);
>   		eb_batch_release(&batch);