OOM killer invoked while still one forth of mem is available

[Christophe LEROY]

Le 26/04/2018 à 15:11, Michal Hocko a écrit :
> On Thu 26-04-18 08:10:30, Christophe LEROY wrote:
>>
>>
>> Le 25/04/2018 à 21:57, David Rientjes a écrit :
>>> On Tue, 24 Apr 2018, christophe leroy wrote:
>>>
>>>> Hi
>>>>
>>>> Allthough there is still about one forth of memory available (7976kB
>>>> among 32MB), oom-killer is invoked and makes a victim.
>>>>
>>>> What could be the reason and how could it be solved ?
>>>>
>>>> [   54.400754] S99watchdogd-ap invoked oom-killer:
>>>> gfp_mask=0x27000c0(GFP_KERNEL_ACCOUNT|__GFP_NOTRACK), nodemask=0,
>>>> order=1, oom_score_adj=0
>>>> [   54.400815] CPU: 0 PID: 777 Comm: S99watchdogd-ap Not tainted
>>>> 4.9.85-local-knld-998 #5
>>>> [   54.400830] Call Trace:
>>>> [   54.400910] [c1ca5d10] [c0327d28] dump_header.isra.4+0x54/0x17c
>>>> (unreliable)
>>>> [   54.400998] [c1ca5d50] [c0079d88] oom_kill_process+0xc4/0x414
>>>> [   54.401067] [c1ca5d90] [c007a5c8] out_of_memory+0x35c/0x37c
>>>> [   54.401220] [c1ca5dc0] [c007d68c] __alloc_pages_nodemask+0x8ec/0x9a8
>>>> [   54.401318] [c1ca5e70] [c00169d4] copy_process.isra.9.part.10+0xdc/0x10d0
>>>> [   54.401398] [c1ca5f00] [c0017b30] _do_fork+0xcc/0x2a8
>>>> [   54.401473] [c1ca5f40] [c000a660] ret_from_syscall+0x0/0x38
>>>
>>> Looks like this is because the allocation is order-1, likely the
>>> allocation of a struct task_struct for a new process on fork.
>>
>> I'm not sure I understand what you mean. The allocation is order 1, yes,
>> does it explains why OOM killer is invoked ?
> 
> Well, not really
> [   54.437414] DMA: 460*4kB (UH) 201*8kB (UH) 121*16kB (UH) 43*32kB (UH) 10*64kB (U) 4*128kB (UH) 0*256kB 0*512kB 0*1024kB 0*2048kB 0*4096kB 0*8192kB = 7912kB`
> 
> You should have enough order-1+ pages to proceed.
> 

So, order is 1 so order - 1 is 0, what's wrong then ? Do the (UH) and 
(U) means anything special ? Otherwise, just above it says 'free:1994', 
so with 1994 pages free I should have enough to proceed, shouldn't I ?

Am I missing something ?

Christophe