Re: [PATCH v4 5/9] xfs: limit the count of a pNFS write allocation to one extent

["Pankaj Raghav (Samsung)" <pankaj.raghav@linux.dev>]

> With pynfs as the client and a 32 GiB XFS, an RW LAYOUTGET at offset 0
> of an empty file for 16 TiB shut the filesystem down: "Corruption of
> in-memory data (0x8) detected at xfs_trans_mod_sb". With this patch it
> gets NFS4ERR_NOSPC, also with XFS_DEBUG, XFS keeps running, and less
> than 8 GiB of the filesystem, one extent's worth, is left free. One
> for 100 GiB ends the same way, where it got NFS4ERR_NOSPC with nothing
> allocated before. One for 16 GiB, which fits, is granted the same
> three extents as before.
> 
> Suggested-by: Darrick J. Wong <djwong@kernel.org>
> Link: https://lore.kernel.org/r/20261008154017.GO2705364@frogsfrogsfrogs
> Suggested-by: Christoph Hellwig <hch@lst.de>
> Link: https://lore.kernel.org/r/20261015060932.GD14035@lst.de
> Fixes: 527851124d10 ("xfs: implement pNFS export operations")
> Cc: stable@vger.kernel.org
> Signed-off-by: Daejun Park <daejun7.park@samsung.com>
> ---
>  fs/xfs/xfs_pnfs.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/fs/xfs/xfs_pnfs.c b/fs/xfs/xfs_pnfs.c
> index 19eb18e129..401a790299 100644
> --- a/fs/xfs/xfs_pnfs.c
> +++ b/fs/xfs/xfs_pnfs.c
> @@ -224,6 +224,14 @@ xfs_fs_map_blocks(
>  		else if (nimaps && imap.br_startblock == HOLESTARTBLOCK)
>  			end_fsb = min(end_fsb, imap.br_startoff +
>  					       imap.br_blockcount);
> +		/*
> +		 * xfs_bmapi_write() maps at most XFS_MAX_BMBT_EXTLEN blocks
> +		 * in one mapping, so don't reserve blocks for more. The
> +		 * range comes from a client, and its block count may not
> +		 * even fit the 32-bit reservation. The rest of the range
> +		 * is mapped by a later call.
> +		 */
> +		end_fsb = min(end_fsb, offset_fsb + XFS_MAX_BMBT_EXTLEN);

Isn't it better we add this in xfs_direct_write_iomap_begin() instead of
adding it in pnfs?

Something like this:

diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
index 329c745e3120..50cf7b0cfc76 100644
--- a/fs/xfs/xfs_iomap.c
+++ b/fs/xfs/xfs_iomap.c
@@ -284,6 +284,7 @@ xfs_iomap_write_direct(
        int                     nr_exts = XFS_IEXT_ADD_NOSPLIT_CNT;
 
        ASSERT(count_fsb > 0);
+       count_fsb = min_t(xfs_filblks_t, count_fsb, XFS_MAX_BMBT_EXTLEN);
 
        resaligned = xfs_aligned_fsb_count(offset_fsb, count_fsb,
                                           xfs_get_extsz_hint(ip));

-- 
Pankaj