linux-kernel - Re: [patch] mm, hugetlb_cgroup: suppress SIGBUS when hugetlb

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Message-ID: <20180528090329.GF1517@dhcp22.suse.cz>
Date:   Mon, 28 May 2018 11:03:29 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     David Rientjes <rientjes@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        Vlastimil Babka <vbabka@...e.cz>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [patch] mm, hugetlb_cgroup: suppress SIGBUS when hugetlb_cgroup
 charge fails

On Fri 25-05-18 15:18:11, David Rientjes wrote:
[...]
> Let's see what Mike and Aneesh say, because they may object to using 
> VM_FAULT_OOM because there's no way to guarantee that we'll come under the 
> limit of hugetlb_cgroup as a result of the oom.  My assumption is that we 
> use VM_FAULT_SIGBUS since oom killing will not guarantee that the 
> allocation can succeed.

Yes. And the lack of hugetlb awareness in the oom killer is another
reason. There is absolutely no reason to kill a task when somebody
misconfigured the hugetlb pool.

> But now a process can get a SIGBUS if its hugetlb 
> pages are not allocatable or its under a limit imposed by hugetlb_cgroup 
> that it's not aware of.  Faulting hugetlb pages is certainly risky 
> business these days...

It's always been and I am afraid it will always be unless somebody
simply reimplements the current code to be NUMA aware for example (it is
just too easy to drain a per NODE reserves...).

> Perhaps the optimal solution for reaching hugetlb_cgroup limits is to 
> induce an oom kill from within the hugetlb_cgroup itself?  Otherwise the 
> unlucky process to fault their hugetlb pages last gets SIGBUS.

Hmm, so you expect that the killed task would simply return pages to the
pool? Wouldn't that require to have a hugetlb cgroup OOM killer that
would only care about hugetlb reservations of tasks? Is that worth all
the effort and the additional code?
-- 
Michal Hocko
SUSE Labs