Re: [PATCH 0/4] use up highorder free pages before OOM

From: Michal Hocko
Date: Mon Oct 10 2016 - 03:59:19 EST

On Sat 08-10-16 00:04:25, Minchan Kim wrote:
> I can show other log which reserve greater than 1%. See the DMA32 zone
> free pages. It was GFP_ATOMIC allocation so it's different with I posted
> but important thing is VM can reserve memory greater than 1% by the race
> which was really what we want.
> in:imklog: page allocation failure: order:0, mode:0x2280020(GFP_ATOMIC|__GFP_NOTRACK)
> DMA: 7*4kB (UE) 3*8kB (UH) 1*16kB (M) 0*32kB 2*64kB (U) 1*128kB (M) 1*256kB (U) 0*512kB 1*1024kB (U) 1*2048kB (U) 1*4096kB (H) = 7748kB
> DMA32: 10*4kB (H) 3*8kB (H) 47*16kB (H) 38*32kB (H) 5*64kB (H) 1*128kB (H) 2*256kB (H) 3*512kB (H) 3*1024kB (H) 3*2048kB (H) 4*4096kB (H) = 30128kB

Yes, this sounds like a bug. Please add this information to the patch
which aims to fix the misaccounting.

> > So while I do agree that potential issues - misaccounting and others you
> > are addressing in the follow up patch - are good to fix but I believe that
> > draining last 19M is not something that would reliably get you over the
> > edge. Your workload (93% of memory sitting on anon LRU with swap full)
> > simply doesn't fit into the amount of memory you have available.
> What happens if the workload fit into additional 19M memory?
> I admit my testing aimed for proving the problem but with this patchset,
> there is no OOM killing with many free pages and the number of OOM was
> reduced highly. It is definitely better than old.
> Please don't ignore 1% memory in embedded system. 20M memory in 2G system,
> If we can use those for zram, it is 60~80M memory via compression.
> You should know how many engineers try to reduce 1M of their driver to
> cost down of the product, seriously.

I am definitely not ignoring neither embedded systems nor 1% of the
memory that might really matter. I just wanted to point out that being
that close to OOM usually blows up later or starts trashing very soon.
It is true that a particular workload might benefit from ever last
allocatable page in the system but it would be better to mention all
that in the changelog.
Michal Hocko