linux-kernel - Re: [patch -mm 4/9 v2] oom: remove compulsory panic_on

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Date:	Wed, 17 Feb 2010 12:21:06 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	Nick Piggin <npiggin@...e.de>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Lubos Lunak <l.lunak@...e.cz>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [patch -mm 4/9 v2] oom: remove compulsory panic_on_oom mode

On Tue, 16 Feb 2010 18:58:17 -0800 (PST)
David Rientjes <rientjes@...gle.com> wrote:

> On Wed, 17 Feb 2010, KAMEZAWA Hiroyuki wrote:
> 
> > > We want to lock all populated zones with ZONE_OOM_LOCKED to avoid 
> > > needlessly killing more than one task regardless of how many memcgs are 
> > > oom.
> > > 
> > Current implentation archive what memcg want. Why remove and destroy memcg ?
> > 
> 
> I've updated my patch to not take ZONE_OOM_LOCKED for any zones on memcg 
> oom.  I'm hoping that you will add sysctl_panic_on_oom == 2 for this case 
> later, however.
> 
I'll write panic_on_oom for memcg, later. 

> > What I mean is
> >  - What VM_FAULT_OOM means is not "memory is exhausted" but "something is exhausted".
> > 
> > For example, when hugepages are all used, it may return VM_FAULT_OOM.
> > Especially when nr_overcommit_hugepage == usage_of_hugepage, it returns VM_FAULT_OOM.
> > 
> 
> The hugetlb case seems to be the only misuse of VM_FAULT_OOM where it 
> doesn't mean we simply don't have the memory to handle the page fault, 
> i.e. your earlier "memory is exhausted" definition.  That was handled well 
> before calling out_of_memory() by simply killing current since we know it 
> is faulting hugetlb pages and its resource is limited.
> 
> We could pass the vma to pagefault_out_of_memory() and simply kill current 
> if its killable and is_vm_hugetlb_page(vma).
> 

No. hugepage is not only case.
You may not read but we annoyed i915's driver bug recently and it was clearly
misuse of VM_FAULT_OOM. Then, we got many reports of OOM killer in these months.
(thanks to Kosaki about this.)

quick glance around core codes...
 - HUGEPAGE at el. should return some VM_FAULT_NO_RESOUECE rather than VM_FAULT_OOM.
 - filemap.c's VM_FAULT_OOM shoudn't call page_fault_oom_kill because it has already
   called oom_killer if it can. 
 - about relayfs, is VM_FAULT_OOM should be BUG_ON()...
 - filemap_xip.c return VM_FAULT_OOM....but it doesn't seem to be OOM..
   just like VM_FAULT_NO_VALID_PAGE_FOUND. (But I'm not familiar with this area.)
 - fs/buffer.c 's VM_FAULT_OOM is returned oom-killer is called.
 - shmem.c's VM_FAULT_OOM is retuned oom-killer is called.

i915's VM_FAULT_OOM is miterious but I can't find whether its real OOM or just shortage
of is own resource. I think VM_FAULT_NO_RESOUCE should be added.


Thanks,
-Kame


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