Re: [PATCH 2/3] ntfs: restart the zone search when the allocation hint fails

[liubaolin <liubaolin12138@163.com>]

在 2026/9/27 18:57, Matthias Goergens 写道:
> When ntfs_cluster_alloc() is given a start_lcn, it first tries the
> clusters from there on.  If that does not satisfy the request, it moves
> on to the zone's current position, but keeps the pass, the zone_end and
> the has_guess state it had.  That loses free clusters in two ways.  If
> the search had already moved on to pass 2, whose range ends at the hint,
> it scans only from the zone position to the hint and misses every free
> cluster between the start of the zone and the zone position.  If no
> cluster had been tested yet, has_guess is still set, so the cluster at
> the zone position is tried as if it had been the hint, and when that
> cluster is in use the rest of the bitmap buffer is skipped.
> 
> Both happen when the hint lies at or past the end of the volume, which
> ntfs_attr_map_cluster() produces when it extrapolates from the last
> allocated run across a hole.  The allocator then finds nothing, shrinks
> the MFT zone to nothing trying to satisfy the request, and fails with
> -ENOSPC.
> 
> To reproduce on a 128 MiB volume with 4 KiB clusters, extend two files
> in turn by one cluster at a time, alternating between fallocate(),
> write() past EOF and truncate() up, so that the runs of each file are
> separated by holes.  When the volume is full, truncate both files back
> to 1 MiB and start again.  During the second round a one-cluster
> fallocate() fails with ENOSPC while 40 MiB is free.
> 
> Start the zone over as if no hint had been given.
> 
> Fixes: 11ccc9107dc4 ("ntfs: update runlist handling and cluster allocator")
> Signed-off-by: Matthias Goergens <matthias.goergens@gmail.com>
> ---
>   fs/ntfs/lcnalloc.c | 18 +++++++++++++++---
>   1 file changed, 15 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/ntfs/lcnalloc.c b/fs/ntfs/lcnalloc.c
> index 0d6cd08ee2e76..30faf422a5424 100644
> --- a/fs/ntfs/lcnalloc.c
> +++ b/fs/ntfs/lcnalloc.c
> @@ -506,13 +506,25 @@ struct runlist_element *ntfs_cluster_alloc(struct ntfs_volume *vol, const s64 st
>   		}
>   
>   		if (!used_zone_pos) {
> +			/*
> +			 * Leaving @start_lcn for the zone position starts the
> +			 * zone over as if no hint had been given, even if the
> +			 * search had already reached pass 2, whose range ends
> +			 * at @start_lcn.
> +			 */
>   			used_zone_pos = 1;
> -			if (search_zone == 1)
> +			has_guess = 0;
> +			pass = 1;

Hi Matthias,
   Resetting pass, zone_end and has_guess here fixes the missed free 
clusters when falling back from the hint to the zone position.

   However, !used_zone_pos only means that the zone position has not 
been used yet; it does not mean that no clusters have been allocated.
   A partial hint allocation can also reach this block at the end of the 
bitmap buffer. Clearing has_guess then allows a nonadjacent run to be 
appended even when is_contig is true.

   Could we return the existing partial run before resetting the search 
state, like this?

   	if (!used_zone_pos) {
   		if (is_contig && rlpos)
   			goto out;

   		used_zone_pos = 1;
   		has_guess = 0;
   		pass = 1;
   		...
   	}

Thanks,
Baolin.

> +			if (search_zone == 1) {
>   				zone_start = vol->mft_zone_pos;
> -			else if (search_zone == 2)
> +				zone_end = vol->mft_zone_end;
> +			} else if (search_zone == 2) {
>   				zone_start = vol->data1_zone_pos;
> -			else
> +				zone_end = vol->nr_clusters;
> +			} else {
>   				zone_start = vol->data2_zone_pos;
> +				zone_end = vol->mft_zone_start;
> +			}
>   
>   			if (!zone_start || zone_start == vol->mft_zone_start ||
>   			    zone_start == vol->mft_zone_end)