linux-ext4 - Re: [PATCH v5 04/12] fscrypt: Drop d_revalidate for valid dentries during lookup

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Date: Wed, 31 Jan 2024 15:35:40 -0300
From: Gabriel Krisman Bertazi <krisman@...e.de>
To: Eric Biggers <ebiggers@...nel.org>
Cc: viro@...iv.linux.org.uk,  jaegeuk@...nel.org,  tytso@....edu,
  amir73il@...il.com,  linux-ext4@...r.kernel.org,
  linux-f2fs-devel@...ts.sourceforge.net,  linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v5 04/12] fscrypt: Drop d_revalidate for valid dentries
 during lookup

Eric Biggers <ebiggers@...nel.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?


-- 
Gabriel Krisman Bertazi