From: Han Young <hanyang.tony@xxxxxxxxxxxxx> [ Upstream commit 3a3065352f73381d3a1aa0ccab44aec3a5a9b365 ] fattr4_numlinks is a recommended attribute, so the client should emulate it even if the server doesn't support it. In decode_attr_nlink function in nfs4xdr.c, nlink is initialized to 1. However, this default value isn't set to the inode due to the check in nfs_fhget. So if the server doesn't support numlinks, inode's nlink will be zero, the mount will fail with error "Stale file handle". Set the nlink to 1 if the server doesn't support it. Signed-off-by: Han Young <hanyang.tony@xxxxxxxxxxxxx> Signed-off-by: Anna Schumaker <anna.schumaker@xxxxxxxxxx> Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx> --- Based on my analysis of the commit and the kernel source code, here is my determination: **YES** This commit should be backported to stable kernel trees. Here's my extensive analysis: ## Critical Bug Fix Analysis ### 1. **Root Cause Understanding** The commit addresses a critical issue where NFSv4 mounts fail with "Stale file handle" errors when the server doesn't support the `fattr4_numlinks` (NLINK) attribute. Looking at the code: - In `/home/sasha/linux/fs/nfs/nfs4xdr.c:3969`, the `decode_attr_nlink` function initializes `*nlink = 1` as a default - However, in `/home/sasha/linux/fs/nfs/inode.c:556-559`, the current logic only sets the inode's nlink if `fattr->valid & NFS_ATTR_FATTR_NLINK` is true - When the server doesn't support numlinks, the `fattr->valid` flag isn't set, so the inode's nlink remains 0 (from initial inode allocation) ### 2. **Impact of Zero nlink** From `/home/sasha/linux/fs/nfs/dir.c:1578-1582`, I can see the critical check: ```c if (inode->i_nlink > 0 || (inode->i_nlink == 0 && test_bit(NFS_INO_PRESERVE_UNLINKED, &NFS_I(inode)->flags))) return 0; else return -ESTALE; ``` When `i_nlink == 0` and the `NFS_INO_PRESERVE_UNLINKED` flag isn't set, NFS returns `-ESTALE`, causing "Stale file handle" errors and mount failures. ### 3. **Backport Suitability Criteria** **✅ Fixes important user-affecting bug**: Mount failures prevent basic filesystem functionality **✅ Small and contained fix**: The change adds only 2 lines: ```c else if (fattr_supported & NFS_ATTR_FATTR_NLINK) nfs_set_cache_invalid(inode, NFS_INO_INVALID_NLINK); +else + set_nlink(inode, 1); ``` **✅ Minimal regression risk**: Setting nlink=1 as fallback is safe and follows the NFSv4 spec (fattr4_numlinks is a "recommended" attribute, so emulation is appropriate) **✅ No architectural changes**: This is a targeted fix within existing error handling logic **✅ Follows stable tree rules**: It's an important bugfix with minimal risk ### 4. **Comparison with Similar Commits** Unlike the provided similar commits that were marked "NO" (which involved more complex nlink management, cache invalidation changes, or feature additions), this commit: - Fixes a fundamental compatibility issue - Prevents complete mount failures - Is minimal and defensive - Addresses server interoperability problems ### 5. **Server Compatibility** This fix ensures NFSv4 clients remain functional with servers that don't fully implement all recommended attributes, which is critical for real-world deployments where server implementations vary. The commit directly prevents mount failures that would otherwise require server-side fixes or workarounds, making it essential for stable kernel trees where users expect reliable NFS functionality. fs/nfs/inode.c | 2 ++ 1 file changed, 2 insertions(+) diff --git a/fs/nfs/inode.c b/fs/nfs/inode.c index 06230baaa554e..419d98cf9e29f 100644 --- a/fs/nfs/inode.c +++ b/fs/nfs/inode.c @@ -555,6 +555,8 @@ nfs_fhget(struct super_block *sb, struct nfs_fh *fh, struct nfs_fattr *fattr) set_nlink(inode, fattr->nlink); else if (fattr_supported & NFS_ATTR_FATTR_NLINK) nfs_set_cache_invalid(inode, NFS_INO_INVALID_NLINK); + else + set_nlink(inode, 1); if (fattr->valid & NFS_ATTR_FATTR_OWNER) inode->i_uid = fattr->uid; else if (fattr_supported & NFS_ATTR_FATTR_OWNER) -- 2.39.5
