Re: [PATCH] nfsd: do not return overlapping extents in a block layout

[Christoph Hellwig]

On Wed, Oct 07, 2026 at 11:03:01AM +0900, Daejun Park via B4 Relay wrote:
> From: Daejun Park <daejun7.park@xxxxxxxxxxx>
> 
> Since commit cc6c40e09d7b ("NFSD/blocklayout: Support multiple extents
> per LAYOUTGET"), nfsd4_block_proc_layoutget() calls ->map_blocks once
> per extent of a LAYOUTGET, each time for the range left after the
> previous extent. nfsd4_block_map_extent() takes a mapping that starts at
> the offset asked for or below it, but only the first extent may start
> below it. A later extent that starts below its offset overlaps the
> extents before it, which RFC 5663 section 2.3.1 does not allow. The
> Linux client rejects such a layout (verify_extent() returns -EIO), and
> the I/O that needed it fails. The block and SCSI layouts share this
> code.
> 
> XFS, the only ->map_blocks implementation, used to map the whole extent
> that contains the offset. Together with one call per extent, that gave
> overlapping layouts in v6.19 and v7.0: allocating the blocks for one
> extent of a write layout could merge the extent that the next call
> starts in with the unwritten extents before it. Since commit
> 36ca6f11424a ("xfs: fix overlapping extents returned for pNFS
> LAYOUTGET"), XFS does not map below the offset, so this patch changes
> nothing with current XFS. It only stops nfsd from relying on that: trim
> each extent after the first so that it starts at the offset asked for,
> and move its volume offset by the same amount unless it is a NONE_DATA
> extent, which has none.
> 
> nfsd leaves the end of a mapping alone, as only the filesystem knows
> where it should end. It does need a mapping that contains the offset
> asked for, which RFC 5663 section 2.3.1 also requires of the first
> extent. A mapping that does not would leave a gap in the layout or make
> the length computed from it wrap around, so warn and return
> NFS4ERR_LAYOUTUNAVAILABLE; the client then does the I/O through the
> metadata server. iomap_iter_done() has the same check, as a
> WARN_ON_ONCE(), for ->iomap_begin(), and nfsd4_block_map_extent()
> already warns and fails this way for a mapping of an unexpected type.
> Document in exportfs_block.h that the mapping must contain the offset.
> 
> On a test kernel whose xfs_fs_map_blocks() maps with XFS_BMAPI_ENTIRE
> and does not trim, as XFS did before that commit, a write layout over a
> hole between two unwritten blocks comes back as 0+4096, 4096+4096 and
> 0+12288, and the pynfs test BLOCK5 fails in five runs out of five.
> fstests generic/075, 091 and 263 over the block layout fail with an EIO
> or a zero-length O_DIRECT write, and bl_alloc_lseg() on the client
> returns -EIO three times. With this patch on top, the third extent is
> 8192+4096, BLOCK5 passes in five runs out of five, generic/091 and 263
> pass, generic/075 fails with the fsx "Size error" that it also fails
> with on nfsd-testing, and bl_alloc_lseg() returns no error.

Looks good:

Reviewed-by: Christoph Hellwig <hch@xxxxxx>

But given that you're pretty active in this part of nfsd now, can you
also look into my earlier suggestion to allow a single map_blocks
return multiple mappings?  That solves more of the root cause of
creating incoherencies.