Re: [PATCH v5 04/12] fscrypt: Drop d_revalidate for valid dentries during lookup

[Gabriel Krisman Bertazi <krisman@suse.de>]

Eric Biggers <ebiggers@kernel.org> writes:

> On Wed, Jan 31, 2024 at 03:35:40PM -0300, Gabriel Krisman Bertazi wrote:
>> Eric Biggers <ebiggers@kernel.org> writes:
>> 
>> > On Mon, Jan 29, 2024 at 05:43:22PM -0300, Gabriel Krisman Bertazi wrote:
>> >> Unencrypted and encrypted-dentries where the key is available don't need
>> >> to be revalidated with regards to fscrypt, since they don't go stale
>> >> from under VFS and the key cannot be removed for the encrypted case
>> >> without evicting the dentry.  Mark them with d_set_always_valid, to
>> >
>> > "d_set_always_valid" doesn't appear in the diff itself.
>> >
>> >> diff --git a/include/linux/fscrypt.h b/include/linux/fscrypt.h
>> >> index 4aaf847955c0..a22997b9f35c 100644
>> >> --- a/include/linux/fscrypt.h
>> >> +++ b/include/linux/fscrypt.h
>> >> @@ -942,11 +942,22 @@ static inline int fscrypt_prepare_rename(struct inode *old_dir,
>> >>  static inline void fscrypt_prepare_lookup_dentry(struct dentry *dentry,
>> >>  						 bool is_nokey_name)
>> >>  {
>> >> -	if (is_nokey_name) {
>> >> -		spin_lock(&dentry->d_lock);
>> >> +	spin_lock(&dentry->d_lock);
>> >> +
>> >> +	if (is_nokey_name)
>> >>  		dentry->d_flags |= DCACHE_NOKEY_NAME;
>> >> -		spin_unlock(&dentry->d_lock);
>> >> +	else if (dentry->d_flags & DCACHE_OP_REVALIDATE &&
>> >> +		 dentry->d_op->d_revalidate == fscrypt_d_revalidate) {
>> >> +		/*
>> >> +		 * Unencrypted dentries and encrypted dentries where the
>> >> +		 * key is available are always valid from fscrypt
>> >> +		 * perspective. Avoid the cost of calling
>> >> +		 * fscrypt_d_revalidate unnecessarily.
>> >> +		 */
>> >> +		dentry->d_flags &= ~DCACHE_OP_REVALIDATE;
>> >>  	}
>> >> +
>> >> +	spin_unlock(&dentry->d_lock);
>> >
>> > This makes lookups in unencrypted directories start doing the
>> > spin_lock/spin_unlock pair.  Is that really necessary?
>> >
>> > These changes also make the inline function fscrypt_prepare_lookup() very long
>> > (when including the fscrypt_prepare_lookup_dentry() that's inlined into it).
>> > The rule that I'm trying to follow is that to the extent that the fscrypt helper
>> > functions are inlined, the inline part should be a fast path for unencrypted
>> > directories.  Encrypted directories should be handled out-of-line.
>> >
>> > So looking at the original fscrypt_prepare_lookup():
>> >
>> > 	static inline int fscrypt_prepare_lookup(struct inode *dir,
>> > 						 struct dentry *dentry,
>> > 						 struct fscrypt_name *fname)
>> > 	{
>> > 		if (IS_ENCRYPTED(dir))
>> > 			return __fscrypt_prepare_lookup(dir, dentry, fname);
>> >
>> > 		memset(fname, 0, sizeof(*fname));
>> > 		fname->usr_fname = &dentry->d_name;
>> > 		fname->disk_name.name = (unsigned char *)dentry->d_name.name;
>> > 		fname->disk_name.len = dentry->d_name.len;
>> > 		return 0;
>> > 	}
>> >
>> > If you could just add the DCACHE_OP_REVALIDATE clearing for dentries in
>> > unencrypted directories just before the "return 0;", hopefully without the
>> > spinlock, that would be good.  Yes, that does mean that
>> > __fscrypt_prepare_lookup() will have to handle it too, for the case of dentries
>> > in encrypted directories, but that seems okay.
>> 
>> ok, will do.  IIUC, we might be able to do without the d_lock
>> provided there is no store tearing.
>> 
>> But what was the reason you need the d_lock to set DCACHE_NOKEY_NAME
>> during lookup?  Is there a race with parallel lookup setting d_flag that
>> I couldn't find? Or is it another reason?
>
> d_flags is documented to be protected by d_lock.  So for setting
> DCACHE_NOKEY_NAME, fs/crypto/ just does the safe thing of taking d_lock.  I
> never really looked into whether the lock can be skipped there (i.e., whether
> anything else can change d_flags while ->lookup is running), since this code
> only ran for no-key names, for which performance isn't really important.

Yes, I was looking for the actual race that could happen here, and
couldn't find one. As far as I understand it, the only thing that could
see the dentry during a lookup would be a parallel lookup, but those
will be held waiting for completion in d_alloc_parallel, and won't touch
d_flags.  Currently, right after this code, we call d_set_d_op() in
generic_set_encrypted_ci_d_ops(), which will happily write d_flags without
the d_lock. If this is a problem here, we have a problem there.

What I really don't want to do is keep the lock for DCACHE_NOKEY_NAME,
but drop it for unsetting DCACHE_OP_REVALIDATE right in the same field,
without a good reason.  I get the argument that unencrypted
dentries are a much hotter path and we care more.  But the locking rules
of ->d_lookup don't change for both cases.

So, I'd rather drop the d_lock entirely in this path, not only for the
hunk I'm proposing.  It would be good to get an actual confirmation from
Al or Christian, though.

CC'ing Christian.

-- 
Gabriel Krisman Bertazi