[Bug 1686679] [NEW] [SRU] Ubuntu16.04 : autofs is extremely long with large number of direct maps

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[Impact]

autofs service in xenial takes extremely long time since it parses the
mount table /etc/mtab in contained_in_local_fs() to determine if the mount
patch would be created on a local file system, that means startup process will read /etc/mtab for every mount, so the performance is extremely low. The customer is complaining about this point, we need to backport the
following upstream patch.

https://git.kernel.org/pub/scm/linux/storage/autofs/autofs.git/commit/?id=67e7d613a4b09eeffc57ab44a7acb52027d897b2

This fix patch removes contained_in_local_fs, and use fs.f_type in is_remote_fstype() to determine if the mount patch would be created on a
 local file system instead.

[Test Case]

 * Create an autofs direct mount map with large direct mappings (eg:
8k), below is the test script I used.

echo "/-      auto.direct        --timeout 60" >> /etc/auto.master
END=8000
for i in $(seq 1 $END); do echo $i; echo "/test/samba/test"$i" -fstype=cifs,rw,username=hua,password=password ://192.168.99.169/share" >> /etc/auto.direct; done

 * Start autofs service to see if it will take too much time, below is my
 result before/after applying the fix patch.

root at node1:~# time service autofs start
real	3m5.481s
user	0m0.256s
sys	0m0.080s
root at node1:~# time service autofs start
real	0m0.833s
user	0m0.000s
sys	0m0.004s

[Regression Potential]

* We are well aware of the principle of this fix patch - avoiding parsing
the mount table to improve startup time, so seems infinitely better for
all cases.

* This fix from upstream has been backported into Redhat as well, and both
 me and customer have positive test results with automount start timings.

* What releases to fix

$ git tag --contains 67e7d613a4b09eeffc57ab44a7acb52027d897b2
release_5_1_2

$ rmadison autofs5
 autofs5 | 5.1.1-1ubuntu3 | xenial | all
 autofs5 | 5.1.1-1ubuntu3 | yakkety | all
 autofs5 | 5.1.2-1ubuntu1 | zesty | all
 autofs5 | 5.1.2-1ubuntu1 | artful | all

$ rmadison -u debian autofs5
 autofs5    | 5.1.2-1        | unstable        | all

- Ubuntu : Only Xenial and Yakkety are affected.
Zesty and Artful already has the upstream fix because they are at version 5.1.2.

- Debian : unstable also has the fix already.

[Other Info]

* Redhat [1] also already backported the following patch, see: https://bugzilla.redhat.com/show_bug.cgi?id=1440769
* This mailing list also discussed this problem, see: http://www.spinics.net/lists/autofs/msg01161.html

** Affects: autofs (Ubuntu)
     Importance: Medium
         Status: Fix Released

** Affects: autofs (Ubuntu Xenial)
     Importance: Medium
     Assignee: Hua Zhang (zhhuabj)
         Status: Fix Committed

** Affects: autofs (Ubuntu Yakkety)
     Importance: Medium
     Assignee: Hua Zhang (zhhuabj)
         Status: Fix Committed


** Tags: sts sts-sponsor-done sts-sru-needed verification-done-xenial verification-done-yakkety
-- 
[SRU] Ubuntu16.04 : autofs is extremely long with large number of direct maps
https://bugs.launchpad.net/bugs/1686679
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