Re: [PATCH v5 0/7] hugetlb_cgroup: Add hugetlb_cgroup reservation limits

[Mike Kravetz]

On 9/27/19 3:51 PM, Mina Almasry wrote:
> On Fri, Sep 27, 2019 at 2:59 PM Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
>>
>> On 9/26/19 5:55 PM, Mina Almasry wrote:
>>> Provided we keep the existing controller untouched, should the new
>>> controller track:
>>>
>>> 1. only reservations, or
>>> 2. both reservations and allocations for which no reservations exist
>>> (such as the MAP_NORESERVE case)?
>>>
>>> I like the 'both' approach. Seems to me a counter like that would work
>>> automatically regardless of whether the application is allocating
>>> hugetlb memory with NORESERVE or not. NORESERVE allocations cannot cut
>>> into reserved hugetlb pages, correct?
>>
>> Correct.  One other easy way to allocate huge pages without reserves
>> (that I know is used today) is via the fallocate system call.
>>
>>>                                       If so, then applications that
>>> allocate with NORESERVE will get sigbused when they hit their limit,
>>> and applications that allocate without NORESERVE may get an error at
>>> mmap time but will always be within their limits while they access the
>>> mmap'd memory, correct?
>>
>> Correct.  At page allocation time we can easily check to see if a reservation
>> exists and not charge.  For any specific page within a hugetlbfs file,
>> a charge would happen at mmap time or allocation time.
>>
>> One exception (that I can think of) to this mmap(RESERVE) will not cause
>> a SIGBUS rule is in the case of hole punch.  If someone punches a hole in
>> a file, not only do they remove pages associated with the file but the
>> reservation information as well.  Therefore, a subsequent fault will be
>> the same as an allocation without reservation.
>>
> 
> I don't think it causes a sigbus. This is the scenario, right:
> 
> 1. Make cgroup with limit X bytes.
> 2. Task in cgroup mmaps a file with X bytes, causing the cgroup to get charged
> 3. A hole of size Y is punched in the file, causing the cgroup to get
> uncharged Y bytes.
> 4. The task faults in memory from the hole, getting charged up to Y
> bytes again. But they will be still within their limits.
> 
> IIUC userspace only gets sigbus'd if the limit is lowered between
> steps 3 and 4, and it's ok if it gets sigbus'd there in my opinion.

You are correct.  That was my mistake.  I was still thinking of behavior
for a reservation only cgroup model.  It would behave as you describe
above (no SIGBUS) for a combined reservation/allocate model.
-- 
Mike Kravetz