On Fri, Jul 22 2016, J. Bruce Fields wrote:

On Fri, Jul 22, 2016 at 11:08:17AM +1000, NeilBrown wrote:

quoted

On Fri, Jun 10 2016, fdmanana@kernel.org wrote:

quoted

From: Filipe Manana <redacted>

When we attempt to read an inode from disk, we end up always returning an
-ESTALE error to the caller regardless of the actual failure reason, which
can be an out of memory problem (when allocating a path), some error found
when reading from the fs/subvolume btree (like a genuine IO error) or the
inode does not exists. So lets start returning the real error code to the
callers so that they don't treat all -ESTALE errors as meaning that the
inode does not exists (such as during orphan cleanup). This will also be
needed for a subsequent patch in the same series dealing with a special
fsync case.

Signed-off-by: Filipe Manana <redacted>

SNIP

quoted

@@ -5594,7 +5602,8 @@ struct inode *btrfs_iget(struct super_block *s, struct btrfs_key *location,
 		} else {
 			unlock_new_inode(inode);
 			iput(inode);
-			inode = ERR_PTR(-ESTALE);
+			ASSERT(ret < 0);
+			inode = ERR_PTR(ret < 0 ? ret : -ESTALE);
 		}

Just a heads-up.  This change breaks NFS :-(

The change in error code percolates up the call chain:

 nfs4_pufh->fh_verify->nfsd_set_fh_dentry->exportfs_decode_fh
    ->btrfs_fh_to_dentry->ntrfs_get_dentry->btrfs_iget

and nfsd returns NFS4ERR_NOENT to the client instead of NFS4ERR_STALE,
and the client doesn't handle that quite the same way.

This doesn't mean that the change is wrong, but it could mean we need to
fix something else in the path to sanitize the error code.

nfsd_set_fh_dentry already has

	error = nfserr_stale;
	if (PTR_ERR(exp) == -ENOENT)
		return error;

	if (IS_ERR(exp))
		return nfserrno(PTR_ERR(exp));

for a different error case, so duplicating that would work, but I doubt
it is best.  At the very least we should check for valid errors, not
specific invalid ones.

Bruce: do you have an opinion where we should make sure that PUTFH (and
various other requests) returns a valid error code?

Uh, I guess not.  Maybe exportfs_decode_fh?

Though my kneejerk reaction is to be cranky and wonder why btrfs
suddenly needs a different convention for decode_fh().

I can certainly agree with that perspective, though it would be
appropriate in that case to make sure we document the requirements for
fh_to_dentry (the current spelling for 'decode_fh').  So I went looking
for documentation and found:

 * fh_to_dentry:
 *    @fh_to_dentry is given a &struct super_block (@sb) and a file handle
 *    fragment (@fh, @fh_len). It should return a &struct dentry which refers
 *    to the same file that the file handle fragment refers to.  If it cannot,
 *    it should return a %NULL pointer if the file was found but no acceptable
 *    &dentries were available, or an %ERR_PTR error code indicating why it
 *    couldn't be found (e.g. %ENOENT or %ENOMEM).  Any suitable dentry can be
 *    returned including, if necessary, a new dentry created with d_alloc_root.
 *    The caller can then find any other extant dentries by following the
 *    d_alias links.
 *

So the new btrfs code is actually conformant!!
That documentation dates back to 2002 when I wrote it....
And it looks like ENOENT wasn't handled correctly then :-(

I suspect anything that isn't ENOMEM should be converted to ESTALE.
ENOMEM causes the client to be asked to retry the request later.

Does this look reasonable to you?
(Adding Christof as he as contributed a lot to exportfs)

If there is agreement I'll test and post a proper patch.

Thanks,
NeilBrown

diff --git a/fs/exportfs/expfs.c b/fs/exportfs/expfs.c
index 207ba8d627ca..3527b58cd5bc 100644
--- a/fs/exportfs/expfs.c
+++ b/fs/exportfs/expfs.c

@@ -428,10 +428,10 @@ struct dentry *exportfs_decode_fh(struct vfsmount *mnt, struct fid *fid,
 	if (!nop || !nop->fh_to_dentry)
 		return ERR_PTR(-ESTALE);
 	result = nop->fh_to_dentry(mnt->mnt_sb, fid, fh_len, fileid_type);
-	if (!result)
-		result = ERR_PTR(-ESTALE);
-	if (IS_ERR(result))
-		return result;
+	if (PTR_ERR(result) == -ENOMEM)
+		return ERR_CAST(result)
+	if (IS_ERR_OR_NULL(result))
+		return ERR_PTR(-ESTALE);
 
 	if (d_is_dir(result)) {
 		/*

@@ -541,6 +541,8 @@ struct dentry *exportfs_decode_fh(struct vfsmount *mnt, struct fid *fid,
 
  err_result:
 	dput(result);
+	if (err != -ENOMEM)
+		err = -ESTALE;
 	return ERR_PTR(err);
 }
 EXPORT_SYMBOL_GPL(exportfs_decode_fh);