Re: [PATCH] xfs: truncate should remove all blocks, not just to the end of the page cache

[Christoph Hellwig <hch@infradead.org>]

On Sun, Dec 22, 2019 at 08:36:30AM -0800, Darrick J. Wong wrote:
> From: Darrick J. Wong <darrick.wong@oracle.com>
> 
> xfs_itruncate_extents_flags() is supposed to unmap every block in a file
> from EOF onwards.  Oddly, it uses s_maxbytes as the upper limit to the
> bunmapi range, even though s_maxbytes reflects the highest offset the
> pagecache can support, not the highest offset that XFS supports.
> 
> The result of this confusion is that if you create a 20T file on a
> 64-bit machine, mount the filesystem on a 32-bit machine, and remove the
> file, we leak everything above 16T.  Fix this by capping the bunmapi
> request at the maximum possible block offset, not s_maxbytes.
> 
> Fixes: 32972383ca462 ("xfs: make largest supported offset less shouty")

Why would that fix that commit?  The commit just changed how do derive
the value, but not the value itself.

> diff --git a/fs/xfs/xfs_inode.c b/fs/xfs/xfs_inode.c
> index 401da197f012..eaa85d5933cb 100644
> --- a/fs/xfs/xfs_inode.c
> +++ b/fs/xfs/xfs_inode.c
> @@ -1544,9 +1544,12 @@ xfs_itruncate_extents_flags(
>  	 * possible file size.  If the first block to be removed is
>  	 * beyond the maximum file size (ie it is the same as last_block),
>  	 * then there is nothing to do.
> +	 *
> +	 * We have to free all the blocks to the bmbt maximum offset, even if
> +	 * the page cache can't scale that far.
>  	 */
>  	first_unmap_block = XFS_B_TO_FSB(mp, (xfs_ufsize_t)new_size);
> -	last_block = XFS_B_TO_FSB(mp, mp->m_super->s_maxbytes);
> +	last_block = (1ULL << BMBT_STARTOFF_BITLEN) - 1;
>  	if (first_unmap_block == last_block)
>  		return 0;

That check is now never true.  I think that whole function wants some
attenttion instead.  Kill that whole last_block calculation, switch to
__xfs_bunmapi and pass ULLONG_MAX for the rlen input and just exit the
loop once rlen is 0.