[PATCH] nfsd: don't offer flexfiles layouts when NFSv3 isn't being served

[Jeff Layton <jlayton@kernel.org>]

The flexfiles layout driver hands out a deviceinfo that names this server
as an NFSv3 DS (flexfilelayout.c hardcodes da->version = 3). With v3
disabled, the client can't talk to the DS: it loops on v3 NULL calls that
the server answers with PROG_MISMATCH, and I/O hangs in D state.

Skip LAYOUT_FLEX_FILES in nfsd4_setup_layout_type() unless v3 is enabled.

Reported-by: Benji Wiebe <itsme@benjiwiebe.com>
Closes: https://lore.kernel.org/linux-nfs/c734eeb8-8486-4491-bb4d-b9656cbaffbe@app.fastmail.com/
Fixes: 9b9960a0ca47 ("nfsd: Add a super simple flex file server")
Assisted-by: LLM
Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
 fs/nfsd/nfs4layouts.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/fs/nfsd/nfs4layouts.c b/fs/nfsd/nfs4layouts.c
index 22bcb6d09f70..cef4d40c89a2 100644
--- a/fs/nfsd/nfs4layouts.c
+++ b/fs/nfsd/nfs4layouts.c
@@ -127,10 +127,12 @@ nfsd4_set_deviceid(struct nfsd4_deviceid *id, const struct svc_fh *fhp,
 
 void nfsd4_setup_layout_type(struct svc_export *exp)
 {
+	struct nfsd_net *nn = net_generic(exp->cd->net, nfsd_net_id);
 	struct super_block *sb = exp->ex_path.mnt->mnt_sb;
 	expfs_block_layouts_t block_supported = exportfs_layouts_supported(sb);
 
-	if (IS_ENABLED(CONFIG_NFSD_FLEXFILELAYOUT))
+	if (IS_ENABLED(CONFIG_NFSD_FLEXFILELAYOUT) &&
+	    nfsd_vers(nn, 3, NFSD_TEST))
 		exp->ex_layout_types |= 1 << LAYOUT_FLEX_FILES;
 	if (IS_ENABLED(CONFIG_NFSD_BLOCKLAYOUT) &&
 	    (block_supported & EXPFS_BLOCK_IN_BAND_ID))

---
base-commit: cee9395acd8043be0644b25c34bfa86623f2b935
change-id: 20260904-pnfs-ff-485ac21042a6

Best regards,
-- 
Jeff Layton <jlayton@kernel.org>