linux-kernel - Re: [RESEND PATCH 1/2 -mm] mm: account lazy free pages separately

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Date:   Wed, 14 Aug 2019 21:54:23 -0700
From:   Yang Shi <yang.shi@...ux.alibaba.com>
To:     Vlastimil Babka <vbabka@...e.cz>, Michal Hocko <mhocko@...nel.org>
Cc:     kirill.shutemov@...ux.intel.com, hannes@...xchg.org,
        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 5:55 AM, Vlastimil Babka wrote:
> On 8/12/19 7:00 PM, Yang Shi wrote:
>>> I can see that memcg rss size was the primary problem David was looking
>>> at. But MemAvailable will not help with that, right? Moreover is
>> Yes, but David actually would like to have memcg MemAvailable (the
>> accounter like the global one), which should be counted like the global
>> one and should account per memcg deferred split THP properly.
>>
>>> 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.
>>
>> 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.
> If we know the mapcounts in the moment the deferred split is initiated (I
> suppose there has to be a iteration over all subpages already?), we could get
> the exact number to adjust the counter with, and also store the number somewhere
> (e.g. a unused field in first/second tail page, I think we already do that for
> something). Then in the shrinker we just read that number to adjust the counter
> back. Then we can ignore the subpage mapping changes before shrinking happens,
> they shouldn't change the situation significantly, and importantly we we will be
> safe from counter imbalance thanks to the stored number.

Thanks, I'm going to look into this approach. Thanks for the suggestion 
again.