lkml.org 
[lkml]   [2022]   [Jan]   [24]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
SubjectRe: [PATCH] vfs: Pre-allocate superblock in sget_fc() if !test
From
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

>

\
 
 \ /
  Last update: 2022-01-24 15:59    [W:0.039 / U:0.412 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site