On 02/15/2017 07:05 PM, Brian Foster wrote:

You're in inode reclaim, blocked on a memory allocation for an inode
buffer required to flush a dirty inode. I suppose this means that the
backing buffer for the inode has already been reclaimed and must be
re-read, which ideally wouldn't have occurred before the inode is
flushed.

quoted

But it cannot get memory, because it's low (?). So it stays blocked.

Other processes do the same but they can't get past the mutex in
xfs_reclaim_inodes_nr():

...

quoted

Which finally leads to "Kernel panic - not syncing: Out of memory and no
killable processes..." as no process is able to proceed.

I quickly hacked this:

diff --git a/fs/xfs/xfs_icache.c b/fs/xfs/xfs_icache.c
index 9ef152b..8adfb0a 100644
--- a/fs/xfs/xfs_icache.c
+++ b/fs/xfs/xfs_icache.c

@@ -1254,7 +1254,7 @@ struct xfs_inode *
        xfs_reclaim_work_queue(mp);
        xfs_ail_push_all(mp->m_ail);

-       return xfs_reclaim_inodes_ag(mp, SYNC_TRYLOCK | SYNC_WAIT,

&nr_to_scan);
+       return 0; // xfs_reclaim_inodes_ag(mp, SYNC_TRYLOCK | SYNC_WAIT,
&nr_to_scan);
 }

So you've disabled inode reclaim completely...

I don't think this is correct. I disabled direct / kswapd reclaim.
XFS uses background worker for async reclaim:

http://lxr.free-electrons.com/source/fs/xfs/xfs_icache.c#L178
http://lxr.free-electrons.com/source/fs/xfs/xfs_super.c#L1534

Confirmed by running trace-cmd on a patched kernel:

# trace-cmd record -p function -l xfs_reclaim_inodes -l xfs_reclaim_worker
# # trace-cmd report
CPU 0 is empty
CPU 2 is empty
CPU 3 is empty
CPU 5 is empty
CPU 8 is empty
CPU 10 is empty
CPU 11 is empty
cpus=16
     kworker/12:2-31208 [012] 106450.590216: function: 
xfs_reclaim_worker
     kworker/12:2-31208 [012] 106450.590226: function: 
xfs_reclaim_inodes
     kworker/12:2-31208 [012] 106450.756879: function: 
xfs_reclaim_worker
     kworker/12:2-31208 [012] 106450.756882: function: 
xfs_reclaim_inodes
     kworker/12:2-31208 [012] 106450.920212: function: 
xfs_reclaim_worker
     kworker/12:2-31208 [012] 106450.920215: function: 
xfs_reclaim_inodes
     kworker/12:2-31208 [012] 106451.083549: function: 
xfs_reclaim_worker
     kworker/12:2-31208 [012] 106451.083552: function: 
xfs_reclaim_inodes
     kworker/12:2-31208 [012] 106451.246882: function: 
xfs_reclaim_worker
     kworker/12:2-31208 [012] 106451.246885: function: 
xfs_reclaim_inodes
     kworker/12:2-31208 [012] 106451.413546: function: 
xfs_reclaim_worker
     kworker/12:2-31208 [012] 106451.413548: function: 
xfs_reclaim_inodes
     kworker/12:2-31208 [012] 106451.580215: function: 
xfs_reclaim_worker
     kworker/12:2-31208 [012] 106451.580217: function: 
xfs_reclaim_inodes
     kworker/12:2-31208 [012] 106451.743549: function: 
xfs_reclaim_worker
     kworker/12:2-31208 [012] 106451.743550: function: 
xfs_reclaim_inodes
     kworker/12:2-31208 [012] 106451.906882: function: 
xfs_reclaim_worker
     kworker/12:2-31208 [012] 106451.906885: function: 
xfs_reclaim_inodes
     kworker/12:2-31208 [012] 106452.070216: function: 
xfs_reclaim_worker
     kworker/12:2-31208 [012] 106452.070219: function: 
xfs_reclaim_inodes
      kworker/7:0-14419 [007] 106454.730218: function: 
xfs_reclaim_worker
      kworker/7:0-14419 [007] 106454.730227: function: 
xfs_reclaim_inodes
      kworker/1:0-14025 [001] 106455.340221: function: 
xfs_reclaim_worker
      kworker/1:0-14025 [001] 106455.340225: function: 
xfs_reclaim_inodes

The bz shows you have non-default vm settings such as
'vm.vfs_cache_pressure = 200.' My understanding is that prefers
aggressive inode reclaim, yet the code workaround here is to bypass XFS
inode reclaim. Out of curiousity, have you reproduced this problem using
the default vfs_cache_pressure value (or if so, possibly moving it in
the other direction)?

Yes, we've tried that, it had about 0 influence.

-- 
Alexander Polakov | system software engineer | https://beget.com

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