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@xxxxxxxxxx>
---
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
