linux-kernel - Re: [PATCH v8 2/9] hugetlb_cgroup: add interface for charge/uncharge hugetlb reservations

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Date:   Fri, 8 Nov 2019 16:01:19 -0800
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Mina Almasry <almasrymina@...gle.com>
Cc:     shuah <shuah@...nel.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>
Subject: Re: [PATCH v8 2/9] hugetlb_cgroup: add interface for charge/uncharge
 hugetlb reservations

On 11/8/19 3:48 PM, Mina Almasry wrote:
> On Thu, Nov 7, 2019 at 4:57 PM Mike Kravetz <mike.kravetz@...cle.com> wrote:
>>
>> On 10/29/19 6:36 PM, Mina Almasry wrote:
>>> @@ -22,27 +22,35 @@ struct hugetlb_cgroup;
>>>   * Minimum page order trackable by hugetlb cgroup.
>>>   * At least 3 pages are necessary for all the tracking information.
>>>   */
>>> -#define HUGETLB_CGROUP_MIN_ORDER     2
>>> +#define HUGETLB_CGROUP_MIN_ORDER 3
>>
>> Correct me if misremembering, but I think the reson you changed this was
>> so that you could use page[3].private.  Correct?
>> In that case isn't page[3] the last page of an order 2 allocation?
>> If my understanding is correct, then leave HUGETLB_CGROUP_MIN_ORDER as is
>> and update the preceding comment to say that at least 4 pages are necessary.
>>
> 
> Yes, I just misunderstood what MIN_ORDER means. I'll revert the code change.

But, do update the comment please.

<snip>
>>> @@ -85,18 +89,32 @@ static void hugetlb_cgroup_init(struct hugetlb_cgroup *h_cgroup,
>>>       int idx;
>>>
>>>       for (idx = 0; idx < HUGE_MAX_HSTATE; idx++) {
>>> -             struct page_counter *counter = &h_cgroup->hugepage[idx];
>>>               struct page_counter *parent = NULL;
>>
>> Should we perhaps rename 'parent' to 'fault_parent' to be consistent?
> 
> Yes that makes sense; will do.
> 
>> That makes me think if perhaps the naming in the previous patch should
>> be more explicit.  Make the existing names explicitly contin 'fault' as
>> the new names contain 'reservation'.
>> Just a thought.
>>
> 
> You mean change the names of the actual user-facing files? I'm all for
> better names but that would break existing users that read/write the
> hugetlb_cgroup.2MB.usage_in_bytes/limit_in_bytes users, and so I would
> assume is a no-go.
> 

I was thinking about internal variables/definitions such as:

+enum {
+ /* Tracks hugetlb memory faulted in. */
+ HUGETLB_RES_USAGE,
+ /* Tracks hugetlb memory reserved. */
+ HUGETLB_RES_RESERVATION_USAGE,
+ /* Limit for hugetlb memory faulted in. */
+ HUGETLB_RES_LIMIT,
+ /* Limit for hugetlb memory reserved. */
+ HUGETLB_RES_RESERVATION_LIMIT,
+ /* Max usage for hugetlb memory faulted in. */
+ HUGETLB_RES_MAX_USAGE,
+ /* Max usage for hugetlb memory reserved. */
+ HUGETLB_RES_RESERVATION_MAX_USAGE,
+ /* Faulted memory accounting fail count. */
+ HUGETLB_RES_FAILCNT,
+ /* Reserved memory accounting fail count. */
+ HUGETLB_RES_RESERVATION_FAILCNT,
+ HUGETLB_RES_NULL,
+ HUGETLB_RES_MAX,
+};

But, I guess the existing definitions (such as HUGETLB_RES_LIMIT) correspond
closely to the externally visible name.  In that case, you should leave them
as is and ignore my comment.

<ship>
>>> @@ -126,6 +144,26 @@ static void hugetlb_cgroup_css_free(struct cgroup_subsys_state *css)
>>>       kfree(h_cgroup);
>>>  }
>>>
>>> +static void hugetlb_cgroup_move_parent_reservation(int idx,
>>> +                                                struct hugetlb_cgroup *h_cg)
>>> +{
>>> +     struct hugetlb_cgroup *parent = parent_hugetlb_cgroup(h_cg);
>>> +
>>> +     /* Move the reservation counters. */
>>> +     if (!parent_hugetlb_cgroup(h_cg)) {
>>> +             parent = root_h_cgroup;
>>> +             /* root has no limit */
>>> +             page_counter_charge(
>>> +                     &root_h_cgroup->reserved_hugepage[idx],
>>> +                     page_counter_read(
>>> +                             hugetlb_cgroup_get_counter(h_cg, idx, true)));
>>> +     }
>>> +
>>> +     /* Take the pages off the local counter */
>>> +     page_counter_cancel(
>>> +             hugetlb_cgroup_get_counter(h_cg, idx, true),
>>> +             page_counter_read(hugetlb_cgroup_get_counter(h_cg, idx, true)));
>>> +}
>>
>> I know next to nothing about cgroups and am just comparing this to the
>> existing hugetlb_cgroup_move_parent() routine.  hugetlb_cgroup_move_parent
>> updates the cgroup pointer in each page being moved.  Do we need to do
>> something similar for reservations being moved (move pointer in reservation)?
>>
> 
> Oh, good catch. Yes I need to be doing that. I should probably
> consolidate those routines so the code doesn't miss things like this.

This might get a bit ugly/complicated?  Seems like you will need to examine
all hugetlbfs inodes and vma's mapping those inodes.

-- 
Mike Kravetz