Re: [PATCH] btrfs: fix invalid data space release when truncating block in NOCOW mode

[Qu Wenruo <wqu@suse.com>]

在 2025/5/10 02:35, fdmanana@kernel.org 写道:
> From: Filipe Manana <fdmanana@suse.com>
> 
> If when truncating a block we fail to reserve data space and then we
> proceed anyway because we can do a NOCOW write, if we later get an error
> when trying to get the folio from the inode's mapping, we end up releasing
> data space that we haven't reserved, screwing up the bytes_may_use counter
> from the data space_info, eventually resulting in an underflow when all
> other reservations done by other tasks are released, if any, or right away
> if there are no other reservations at the moment.
> 
> This is because when we get an error when trying to grab the block's folio
> we call btrfs_delalloc_release_space(), which releases metadata (which we
> have reserved) and data (which we haven't reserved).
> 
> Fix this by calling btrfs_delalloc_release_space() only if we did reserve
> data space, that is, if we aren't falling back to NOCOW, meaning the local
> variable @only_release_metadata has a false value, otherwise release only
> metadata by calling btrfs_delalloc_release_metadata().
> 
> Fixes: 6d4572a9d71d ("btrfs: allow btrfs_truncate_block() to fallback to nocow for data space reservation")
> Signed-off-by: Filipe Manana <fdmanana@suse.com>

Reviewed-by: Qu Wenruo <wqu@suse.com>

Thanks,
Qu

> ---
>   fs/btrfs/inode.c | 7 +++++--
>   1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
> index d518acb7b01d..8c1f7196636a 100644
> --- a/fs/btrfs/inode.c
> +++ b/fs/btrfs/inode.c
> @@ -4932,8 +4932,11 @@ int btrfs_truncate_block(struct btrfs_inode *inode, u64 offset, u64 start, u64 e
>   	folio = __filemap_get_folio(mapping, index,
>   				    FGP_LOCK | FGP_ACCESSED | FGP_CREAT, mask);
>   	if (IS_ERR(folio)) {
> -		btrfs_delalloc_release_space(inode, data_reserved, block_start,
> -					     blocksize, true);
> +		if (only_release_metadata)
> +			btrfs_delalloc_release_metadata(inode, blocksize, true);
> +		else
> +			btrfs_delalloc_release_space(inode, data_reserved,
> +						     block_start, blocksize, true);
>   		btrfs_delalloc_release_extents(inode, blocksize);
>   		ret = -ENOMEM;
>   		goto out;