Fix false ereport.fs.zfs.config_cache_write events.

[ab-oe]

Description

On pool import when the old cache file is removed the ereport.fs.zfs.config_cache_write event is generated. Because zpool export always removes cache file it happens every export - import sequence.

Motivation and Context

To fix false cache write failure events on every export-import sequence.

How Has This Been Tested?

1. Create zpool.
2. Export zpool.
3. Import zpool.
4. Check if there is no ereport.fs.zfs.config_cache_write in zpool events.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the ZFS on Linux code style requirements.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.
• All commit messages are properly formatted and contain Signed-off-by.
• Change has been approved by a ZFS on Linux member.

[loli10K]

Because zpool export always removes cache file it happens every export - import sequence.

Is this always true? It doesn't seem to be happening on my setup, the cachefile is still alive and well after zpool export:

root@linux:~# cat /sys/module/zfs/version 
0.7.0-65_g41a4f3f
root@linux:~# zpool export $POOLNAME
root@linux:~# rm -fv /etc/zfs/zpool.cache 
removed ‘/etc/zfs/zpool.cache’
root@linux:~# zpool import -d /var/tmp/ $POOLNAME
root@linux:~# zpool list $POOLNAME
NAME       SIZE  ALLOC   FREE  EXPANDSZ   FRAG    CAP  DEDUP  HEALTH  ALTROOT
testpool    48M   106K  47.9M         -     3%     0%  1.00x  ONLINE  -
root@linux:~# md5sum /etc/zfs/zpool.cache
25efb86eaa0624bf91bda1141cf5926b  /etc/zfs/zpool.cache
root@linux:~# zpool export $POOLNAME
root@linux:~# zpool list
no pools available
root@linux:~# md5sum /etc/zfs/zpool.cache
25efb86eaa0624bf91bda1141cf5926b  /etc/zfs/zpool.cache
root@linux:~# 

[ab-oe]

@loli10K what kernel do you use? Since 4.2 there is problem that the cache is never removed. I reported issue and proposed bugfix here: openzfs/spl#648

[loli10K]

@ab-oe thanks, that explains it, i use kernel 4.6.4 on this box. I was in fact testing this PR but still getting config_cache_write events:

root@linux:~# zpool export $POOLNAME
root@linux:~# rm -fv /etc/zfs/zpool.cache 
removed ‘/etc/zfs/zpool.cache’
root@linux:~# zpool import -d /var/tmp/ $POOLNAME
root@linux:~# zpool list $POOLNAME
NAME       SIZE  ALLOC   FREE  EXPANDSZ   FRAG    CAP  DEDUP  HEALTH  ALTROOT
testpool    48M   106K  47.9M         -     3%     0%  1.00x  ONLINE  -
root@linux:~# md5sum /etc/zfs/zpool.cache
0e2e31596ae31332d38a29822805ec12  /etc/zfs/zpool.cache
root@linux:~# zpool export $POOLNAME
root@linux:~# zpool list
no pools available
root@linux:~# md5sum /etc/zfs/zpool.cache
0e2e31596ae31332d38a29822805ec12  /etc/zfs/zpool.cache
root@linux:~# zpool events
TIME                           CLASS
Sep  8 2017 16:06:48.647968347 sysevent.fs.zfs.pool_destroy
Sep  8 2017 16:06:48.655968339 ereport.fs.zfs.config_cache_write
Sep  8 2017 16:06:48.655968339 sysevent.fs.zfs.config_sync
Sep  8 2017 16:06:48.715968282 sysevent.fs.zfs.history_event
Sep  8 2017 16:06:48.739968259 sysevent.fs.zfs.config_sync
Sep  8 2017 16:06:48.739968259 sysevent.fs.zfs.pool_import
Sep  8 2017 16:06:48.743968255 sysevent.fs.zfs.history_event
Sep  8 2017 16:06:48.767968232 sysevent.fs.zfs.config_sync
Sep  8 2017 16:06:48.819968183 sysevent.fs.zfs.pool_destroy
Sep  8 2017 16:06:48.831968171 ereport.fs.zfs.config_cache_write
Sep  8 2017 16:06:48.831968171 sysevent.fs.zfs.config_sync
root@linux:~# 

I now see it's because vn_remove() is returning EACCES instead of ENOENT:

kretprobe_trampoline -> spa_config_write (dp={.scd_link={.next=0xffff880014842cf8, .prev=0xffff880014842cf8}, .scd_path="/etc/zfs/zpool.cache"} nvl=ERROR)
spa_export_common <- spa_config_write (return=13)

[loli10K]

The commit message must be updated to fix the STYLE buildbot warnings, otherwise LGTM

[ab-oe]

@loli10K thanks. I fixed commit message.