linux-kernel - Re: [v2 PATCH -mm] mm: account deferred split THPs into MemAvailable

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Date:   Tue, 27 Aug 2019 14:09:23 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     "Kirill A. Shutemov" <kirill@...temov.name>,
        kirill.shutemov@...ux.intel.com,
        Yang Shi <yang.shi@...ux.alibaba.com>, hannes@...xchg.org,
        rientjes@...gle.com, akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [v2 PATCH -mm] mm: account deferred split THPs into MemAvailable

On Tue 27-08-19 14:01:56, Vlastimil Babka wrote:
> On 8/27/19 1:02 PM, Kirill A. Shutemov wrote:
> > On Tue, Aug 27, 2019 at 08:01:39AM +0200, Michal Hocko wrote:
> >> On Mon 26-08-19 16:15:38, Kirill A. Shutemov wrote:
> >>>
> >>> Unmapped completely pages will be freed with current code. Deferred split
> >>> only applies to partly mapped THPs: at least on 4k of the THP is still
> >>> mapped somewhere.
> >>
> >> Hmm, I am probably misreading the code but at least current Linus' tree
> >> reads page_remove_rmap -> [page_remove_anon_compound_rmap ->\ deferred_split_huge_page even
> >> for fully mapped THP.
> > 
> > Well, you read correctly, but it was not intended. I screwed it up at some
> > point.
> > 
> > See the patch below. It should make it work as intened.
> > 
> > It's not bug as such, but inefficientcy. We add page to the queue where
> > it's not needed.
> 
> But that adding to queue doesn't affect whether the page will be freed
> immediately if there are no more partial mappings, right? I don't see
> deferred_split_huge_page() pinning the page.
> So your patch wouldn't make THPs freed immediately in cases where they
> haven't been freed before immediately, it just fixes a minor
> inefficiency with queue manipulation?

Ohh, right. I can see that in free_transhuge_page now. So fully mapped
THPs really do not matter and what I have considered an odd case is
really happening more often.

That being said this will not help at all for what Yang Shi is seeing
and we need a more proactive deferred splitting as I've mentioned
earlier.

-- 
Michal Hocko
SUSE Labs