Re: [PATCH v3 0/6] hugetlb_cgroup: Add hugetlb_cgroup reservation limits

[Mike Kravetz]

On 9/3/19 10:57 AM, Mike Kravetz wrote:
> On 8/29/19 12:18 AM, Michal Hocko wrote:
>> [Cc cgroups maintainers]
>>
>> On Wed 28-08-19 10:58:00, Mina Almasry wrote:
>>> On Wed, Aug 28, 2019 at 4:23 AM Michal Hocko <mhocko@xxxxxxxxxx> wrote:
>>>>
>>>> On Mon 26-08-19 16:32:34, Mina Almasry wrote:
>>>>>  mm/hugetlb.c                                  | 493 ++++++++++++------
>>>>>  mm/hugetlb_cgroup.c                           | 187 +++++--
>>>>
>>>> This is a lot of changes to an already subtle code which hugetlb
>>>> reservations undoubly are.
>>>
>>> For what it's worth, I think this patch series is a net decrease in
>>> the complexity of the reservation code, especially the region_*
>>> functions, which is where a lot of the complexity lies. I removed the
>>> race between region_del and region_{add|chg}, refactored the main
>>> logic into smaller code, moved common code to helpers and deleted the
>>> duplicates, and finally added lots of comments to the hard to
>>> understand pieces. I hope that when folks review the changes they will
>>> see that! :)
>>
>> Post those improvements as standalone patches and sell them as
>> improvements. We can talk about the net additional complexity of the
>> controller much easier then.
> 
> All such changes appear to be in patch 4 of this series.  The commit message
> says "region_add() and region_chg() are heavily refactored to in this commit
> to make the code easier to understand and remove duplication.".  However, the
> modifications were also added to accommodate the new cgroup reservation
> accounting.  I think it would be helpful to explain why the existing code does
> not work with the new accounting.  For example, one change is because
> "existing code coalesces resv_map entries for shared mappings.  new cgroup
> accounting requires that resv_map entries be kept separate for proper
> uncharging."
> 
> I am starting to review the changes, but it would help if there was a high
> level description.  I also like Michal's idea of calling out the region_*
> changes separately.  If not a standalone patch, at least the first patch of
> the series.  This new code will be exercised even if cgroup reservation
> accounting not enabled, so it is very important than no subtle regressions
> be introduced.

While looking at the region_* changes, I started thinking about this no
coalesce change for shared mappings which I think is necessary.  Am I
mistaken, or is this a requirement?

If it is a requirement, then think about some of the possible scenarios
such as:
- There is a hugetlbfs file of size 10 huge pages.
- Task A has reservations for pages at offset 1 3 5 7 and 9
- Task B then mmaps the entire file which should result in reservations
  at 0 2 4 6 and 8.
- region_chg will return 5, but will also need to allocate 5 resv_map
  entries for the subsequent region_add which can not fail.  Correct?
  The code does not appear to handle this.

BTW, this series will BUG when running libhugetlbfs test suite.  It will
hit this in resv_map_release().

	VM_BUG_ON(resv_map->adds_in_progress);

-- 
Mike Kravetz