linux-kernel - Re: [PATCH v9 1/8] hugetlb_cgroup: Add hugetlb

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Date:   Mon, 13 Jan 2020 14:05:58 -0800
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Mina Almasry <almasrymina@...gle.com>
Cc:     shuah <shuah@...nel.org>, David Rientjes <rientjes@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Greg Thelen <gthelen@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        open list <linux-kernel@...r.kernel.org>, linux-mm@...ck.org,
        linux-kselftest@...r.kernel.org, cgroups@...r.kernel.org,
        Aneesh Kumar <aneesh.kumar@...ux.vnet.ibm.com>,
        Michal Koutný <mkoutny@...e.com>,
        Hillf Danton <hdanton@...a.com>
Subject: Re: [PATCH v9 1/8] hugetlb_cgroup: Add hugetlb_cgroup reservation
 counter

On 1/13/20 1:03 PM, Mina Almasry wrote:
> On Mon, Jan 13, 2020 at 10:44 AM Mike Kravetz <mike.kravetz@...cle.com> wrote:
>>
>> On 12/17/19 3:16 PM, Mina Almasry wrote:
>>> - While usage_in_bytes tracks all *faulted* hugetlb memory,
>>> reservation_usage_in_bytes tracks all *reserved* hugetlb memory and
>>> hugetlb memory faulted in without a prior reservation.
>>
>> To me, this implies that 'faults without reservations' could cause
>> reservation usage to exceed reservation limit?  Or, does the faulting
>> process get a SIGBUS because of the reservation limit even though it
>> is not using reservations?
>>
>> We shall see in subsequent patches.
>>
> 
> The design we went with based on previous discussions is as follows:
> hugetlb pages faulted without a prior reservation get accounted at
> fault time, rather than reservation time, and if the fault causes the
> counter to cross the limit, the charge fails, hence the fault fails,
> hence the process gets sigbus'd.

Ok, sorry I did not recall the design discussion.

> This means that one counter I'm adding here can cover both use cases:
> if the userspace uses MAP_NORESERVE, then their memory is accounted at
> fault time and they may get sigbus'd.

Let's make sure this is clearly documented.  Someone could be surprised
if their application not using reserves gets a SIGBUS because there is a
reserve limit.
-- 
Mike Kravetz