Message-ID: <a8005ff4-4749-8c71-ee4e-7ebda5c49de6@linux.alibaba.com>
Date:   Wed, 14 Aug 2019 21:51:47 -0700
From:   Yang Shi <yang.shi@...ux.alibaba.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     kirill.shutemov@...ux.intel.com, hannes@...xchg.org,
        vbabka@...e.cz, rientjes@...gle.com, akpm@...ux-foundation.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [RESEND PATCH 1/2 -mm] mm: account lazy free pages separately



On 8/14/19 4:08 AM, Michal Hocko wrote:
> On Mon 12-08-19 10:00:17, Yang Shi wrote:
>>
>> On 8/12/19 2:34 AM, Michal Hocko wrote:
>>> On Fri 09-08-19 16:54:43, Yang Shi wrote:
>>>> On 8/9/19 11:26 AM, Yang Shi wrote:
>>>>> On 8/9/19 11:02 AM, Michal Hocko wrote:
>>> [...]
>>>>>> I have to study the code some more but is there any reason why those
>>>>>> pages are not accounted as proper THPs anymore? Sure they are partially
>>>>>> unmaped but they are still THPs so why cannot we keep them accounted
>>>>>> like that. Having a new counter to reflect that sounds like papering
>>>>>> over the problem to me. But as I've said I might be missing something
>>>>>> important here.
>>>>> I think we could keep those pages accounted for NR_ANON_THPS since they
>>>>> are still THP although they are unmapped as you mentioned if we just
>>>>> want to fix the improper accounting.
>>>> By double checking what NR_ANON_THPS really means,
>>>> Documentation/filesystems/proc.txt says "Non-file backed huge pages mapped
>>>> into userspace page tables". Then it makes some sense to dec NR_ANON_THPS
>>>> when removing rmap even though they are still THPs.
>>>>
>>>> I don't think we would like to change the definition, if so a new counter
>>>> may make more sense.
>>> Yes, changing NR_ANON_THPS semantic sounds like a bad idea. Let
>>> me try whether I understand the problem. So we have some THP in
>>> limbo waiting for them to be split and unmapped parts to be freed,
>>> right? I can see that page_remove_anon_compound_rmap does correctly
>>> decrement NR_ANON_MAPPED for sub pages that are no longer mapped by
>>> anybody. LRU pages seem to be accounted properly as well.  As you've
>>> said NR_ANON_THPS reflects the number of THPs mapped and that should be
>>> reflecting the reality already IIUC.
>>>
>>> So the only problem seems to be that deferred THP might aggregate a lot
>>> of immediately freeable memory (if none of the subpages are mapped) and
>>> that can confuse MemAvailable because it doesn't know about the fact.
>>> Has an skewed counter resulted in a user observable behavior/failures?
>> No. But the skewed counter may make big difference for a big scale cluster.
>> The MemAvailable is an important factor for cluster scheduler to determine
>> the capacity.
> But MemAvailable is a very rough estimation. Is relying on it really a
> good measure? I mean there is a lot of reclaimable memory that is not
> reflected there (some fs. internal data structures, networking buffers
> etc.)

Yes, I agree there are other freeable objects not accounted into 
MemAvailable. Their size depends on the workload. But, deferred split 
THPs seems more common with the common workloads. A simple run with 
MariaDB test of mmtest shows it could generate over fifteen thousand 
deferred split THPs (accumulated around 30G in one hour run, 75% of 40G 
memory for my VM). So, it may be worth accounting deferred split THPs in 
MemAvailable.

>
> [...]
>
>>> accounting the full THP correct? What if subpages are still mapped?
>> "Deferred split" definitely doesn't mean they are free. When memory pressure
>> is hit, they would be split, then the unmapped normal pages would be freed.
>> So, when calculating MemAvailable, they are not accounted 100%, but like
>> "available += lazyfree - min(lazyfree / 2, wmark_low)", just like how page
>> cache is accounted.
> Then this is even more dubious IMHO.
>
>> We could get more accurate account, i.e. checking each sub page's mapcount
>> when accounting, but it may change before shrinker start scanning. So, just
>> use the ballpark estimation to trade off the complexity for accurate
>> accounting.
> I do not see much point in fixing up one particular counter when there
> is a whole lot that is even not considered. I would rather live with the
> fact that MemAvailable is only very rough estimate then whack a mole on
> any memory consumer that is freeable directly or indirectly via memory
> reclaim. Because this is likely to be always subtly broken and only
> visible under very specific workloads so there is no way to test for it.

I saw Vlastimil suggested KReclaimable, it seems a good fit. If so we 
don't need create a new counter anymore.

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