Re: [PATCH] vfs: Pre-allocate superblock in sget_fc() if !test

[Waiman Long <longman@redhat.com>]

On 1/24/22 06:37, Christian Brauner wrote:
> On Fri, Jan 21, 2022 at 01:52:55PM -0500, Waiman Long wrote:
>> When the test function is not defined in sget_fc(), we always need
>> to allocate a new superblock. So there is no point in acquiring the
>> sb_lock twice in this case. Optimize the !test case by pre-allocating
>> the superblock first before acquring the lock.
>>
>> Signed-off-by: Waiman Long <longman@redhat.com>
>> ---
>>   fs/super.c | 2 ++
>>   1 file changed, 2 insertions(+)
>>
>> diff --git a/fs/super.c b/fs/super.c
>> index a6405d44d4ca..c2bd5c34a826 100644
>> --- a/fs/super.c
>> +++ b/fs/super.c
>> @@ -520,6 +520,8 @@ struct super_block *sget_fc(struct fs_context *fc,
>>   	struct user_namespace *user_ns = fc->global ? &init_user_ns : fc->user_ns;
>>   	int err;
>>   
>> +	if (!test)
>> +		s = alloc_super(fc->fs_type, fc->sb_flags, user_ns);
> Shouldn't we treat this allocation failure as "fatal" right away and not
> bother taking locks, walking lists and so on? Seems strange to treat it
> as fatal below but not here.
I didn't add the null check because it was a rare case and the check is 
done later on anyway. I do agree that it may look a bit odd. Perhaps I 
should rearrange the code flow as suggested.
>
> (The code-flow in here has always been a bit challenging to follow imho.
> So not super keen to see more special-cases in there. Curious: do you
> see any noticeable performance impact from that lock being taken and
> dropped for the !test case?)

I don't believe there is noticeable performance impact with the !test 
case. The test case, however, can have some noticeable impact if the 
superblock list is long. I am wondering if we just always preallocate 
superblock with the risk that it may get unused and freed later on.

Cheers,
Longman

>