Re: [lkp] [mm, page_alloc] 43993977ba: +88% OOM possibility

[Michal Hocko]

On Fri 30-10-15 16:21:40, Huang, Ying wrote:
> Michal Hocko <mhocko@xxxxxxxxxx> writes:
> 
> > On Wed 28-10-15 13:36:02, kernel test robot wrote:
> >> FYI, we noticed the below changes on
> >> 
> >> https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git master
> >> commit 43993977baecd838d66ccabc7f682342fc6ff635 ("mm, page_alloc:
> >> distinguish between being unable to sleep, unwilling to sleep and
> >> avoiding waking kswapd")
> >> 
> >> We found the OOM possibility increased 88% in a virtual machine with 1G memory.
> >
> > Could you provide dmesg output from this test?
> 
> Sure, Attached.

I can only see a single allocation failure warning:
kworker/u4:1: page allocation failure: order:0, mode:0x2204000

This is obviously a non sleeping allocation with ___GFP_KSWAPD_RECLAIM
set. ___GFP_HIGH (aka access to memory reserves) is not required so a
failure of such an allocation is something to be expected.

[ 2294.616369] Workqueue: btrfs-submit btrfs_submit_helper
[ 2294.616369]  0000000000000000 ffff88000d38f5e0 ffffffff8173f84c 0000000000000000
[ 2294.616369]  ffff88000d38f678 ffffffff811abaee 00000000ffffffff 000000010038f618
[ 2294.616369]  ffff8800584e4148 00000000ffffffff ffff8800584e2f00 0000000000000001
[ 2294.616369] Call Trace:
[ 2294.616369]  [<ffffffff8173f84c>] dump_stack+0x4b/0x63
[ 2294.616369]  [<ffffffff811abaee>] warn_alloc_failed+0x125/0x13d
[ 2294.616369]  [<ffffffff811aecce>] __alloc_pages_nodemask+0x7c9/0x915
[ 2294.616369]  [<ffffffff811ecc7b>] kmem_getpages+0x91/0x155
[ 2294.616369]  [<ffffffff811eef0d>] fallback_alloc+0x1cc/0x24c
[ 2294.616369]  [<ffffffff811eed32>] ____cache_alloc_node+0x151/0x160
[ 2294.616369]  [<ffffffff811ef1ed>] __kmalloc+0xb0/0x134
[ 2294.616369]  [<ffffffff8105d7a5>] ? sched_clock+0x9/0xb
[ 2294.616369]  [<ffffffff8187d929>] ? virtqueue_add+0x78/0x37f
[ 2294.616369]  [<ffffffff8187d929>] virtqueue_add+0x78/0x37f
[ 2294.616369]  [<ffffffff81114f72>] ? __lock_acquire+0x751/0xf55
[ 2294.616369]  [<ffffffff8187dca6>] virtqueue_add_sgs+0x76/0x85

The patch you are referring shouldn't make any change in this path
because alloc_indirect which I expect is the allocation failing here
does:
gfp &= ~(__GFP_HIGHMEM | __GFP_HIGH)

and that came in via b92b1b89a33c ("virtio: force vring descriptors to
be allocated from lowmem").

Are there more failed allocations during the test? The subject would
suggest so.

Thanks!

-- 
Michal Hocko
SUSE Labs
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