Hello. 

we have experience this problem many time, and we try many ways to solve this problem, but always failed.

such as sync && echo 3 > /proc/sys/vm/drop_caches to free more memory, however, after a while, the message come again.

the dmesg show:
May 30 12:35:51 w-openstack21 kernel: XFS: possible memory allocation deadlock in kmem_alloc (mode:0x250)
May 30 12:35:53 w-openstack21 kernel: XFS: possible memory allocation deadlock in kmem_alloc (mode:0x250)
May 30 12:35:55 w-openstack21 kernel: XFS: possible memory allocation deadlock in kmem_alloc (mode:0x250)
May 30 12:35:57 w-openstack21 kernel: XFS: possible memory allocation deadlock in kmem_alloc (mode:0x250)
May 30 12:35:59 w-openstack21 kernel: XFS: possible memory allocation deadlock in kmem_alloc (mode:0x250)

our kernel version is 3.10.0-327.4.5.el7.x86_64

we think this is the kernel memory fragmentation problem, when dmesg show the error message, we check the buddyinfo

[root@w-openstack21 /home/xusiliang]# cat /proc/buddyinfo
Node 0, zone      DMA      1      0      1      0      2      1      1      0      1      1      3
Node 0, zone    DMA32   2190   1898   1367    583    284     81     21      8      5      0      4
Node 0, zone   Normal  85195  90572  56688  32379  16793   5568    930    230    184     11      0
Node 1, zone   Normal  95799 127034  94363  49321  21555   5239    225      2      2      0      0

and I think the kernel have enough free memory
[root@w-openstack21 /home/xusiliang]# free -m
              total        used        free      shared  buff/cache   available
Mem:          64269       50357       11802          65        2109       11757
Swap:         32255         347       31908

This machine is running for qemu,  the file is qemu qcow file type

[root@w-openstack20 /data/nova/instances/7898b630-c4ef-49ab-9ce9-3735e090c282]# file disk
disk: QEMU QCOW Image (v3), has backing file (path /data/nova/instances/_base/3f27393376152ac352b2d85703011e38517e), 429496729600 bytes

we also use xfs_db to check the fragmentation

[root@w-openstack20 /home/xusiliang]# xfs_db -r /dev/mapper/VolGroup00-LogVol04
xfs_db> frag

actual 48421430, ideal 20716, fragmentation factor 99.96%


How can I solve this problem, what do you suggest me to do?

thank you


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