Re: [PATCH 1/3] btrfs: fix wrong start offset for delalloc space release during mmap write

[Qu Wenruo <wqu@suse.com>]

在 2025/5/14 19:59, fdmanana@kernel.org 写道:
> From: Filipe Manana <fdmanana@suse.com>
> 
> If we're doing a mmap write against a folio that has i_size somewhere in
> the middle and we have multiple sectors in the folio, we may have to
> release excess space previously reserved, for the range going from the
> rounded up (to sector size) i_size to the folio's end offset. We are
> calculating the right amount to release and passing it to
> btrfs_delalloc_release_space(), but we are passing the wrong start offset
> of that range - we're passing the folio's start offset instead of the
> end offset, plus 1, of the range for which we keep the reservation. This
> may result in releasing more space then we should and eventually trigger
> an underflow of the data space_info's bytes_may_use counter.
> 
> So fix this by passing the start offset as 'end + 1' instead of
> 'page_start' to btrfs_delalloc_release_space().
> 
> Fixes: d0b7da88f640 ("Btrfs: btrfs_page_mkwrite: Reserve space in sectorsized units")
> Signed-off-by: Filipe Manana <fdmanana@suse.com>

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

Just curious how did you spot this, by pure eyeballing or you have some 
tool exposing this?

Thanks,
Qu> ---
>   fs/btrfs/file.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
> index 660a73b6af90..32aad1b02b01 100644
> --- a/fs/btrfs/file.c
> +++ b/fs/btrfs/file.c
> @@ -1918,7 +1918,7 @@ static vm_fault_t btrfs_page_mkwrite(struct vm_fault *vmf)
>   		if (reserved_space < fsize) {
>   			end = page_start + reserved_space - 1;
>   			btrfs_delalloc_release_space(BTRFS_I(inode),
> -					data_reserved, page_start,
> +					data_reserved, end + 1,
>   					fsize - reserved_space, true);
>   		}
>   	}