OOM killer invoked while still one forth of mem is available

[Michal Hocko]

On Thu 26-04-18 15:28:46, Christophe LEROY wrote:
> 
> 
> 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,

Not sure what you mean by order - 1, maybe I've confused you. order-1
means that the order is 1. So free is not all that important. What you
should look at though is how many order 1+ free blocks are available.

> what's wrong then ? Do the (UH) and (U)
> means anything special ?

Yes, show_migration_types. But I do not see why unmovable pageblocks
should block the allocation. This is a GFP_KERNEL allocation request
essentially - thus unmovable itself. This smells like a bug. We are way
above reserves which could block the allocation.

> Otherwise, just above it says 'free:1994', so with
> 1994 pages free I should have enough to proceed, shouldn't I ?

Not for high order pages as per above...

-- 
Michal Hocko
SUSE Labs