Messages in this thread | | | Date | Mon, 24 Jan 2022 09:58:52 -0500 | Subject | Re: [PATCH] vfs: Pre-allocate superblock in sget_fc() if !test | From | Waiman Long <> |
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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
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