linux-kernel - Re: [PATCH v8 03/10] mm/lru: replace pgdat lru

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Date:   Tue, 14 Apr 2020 12:31:14 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Alex Shi <alex.shi@...ux.alibaba.com>
Cc:     cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, akpm@...ux-foundation.org,
        mgorman@...hsingularity.net, tj@...nel.org, hughd@...gle.com,
        khlebnikov@...dex-team.ru, daniel.m.jordan@...cle.com,
        yang.shi@...ux.alibaba.com, willy@...radead.org,
        shakeelb@...gle.com, Michal Hocko <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Roman Gushchin <guro@...com>,
        Chris Down <chris@...isdown.name>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vlastimil Babka <vbabka@...e.cz>, Qian Cai <cai@....pw>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        David Rientjes <rientjes@...gle.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        swkhack <swkhack@...il.com>,
        "Potyra, Stefan" <Stefan.Potyra@...ktrobit.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Colin Ian King <colin.king@...onical.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Mauro Carvalho Chehab <mchehab+samsung@...nel.org>,
        Peng Fan <peng.fan@....com>,
        Nikolay Borisov <nborisov@...e.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Kirill Tkhai <ktkhai@...tuozzo.com>,
        Yafang Shao <laoar.shao@...il.com>
Subject: Re: [PATCH v8 03/10] mm/lru: replace pgdat lru_lock with lruvec lock

On Tue, Apr 14, 2020 at 12:52:30PM +0800, Alex Shi wrote:
> 在 2020/4/14 上午2:07, Johannes Weiner 写道:
> > Plus, the overhead of tracking is tiny - 512k per G of swap (0.04%).
> > 
> > Maybe we should just delete MEMCG_SWAP and unconditionally track swap
> > entry ownership when the memory controller is enabled. I don't see a
> > good reason not to, and it would simplify the entire swapin path, the
> > LRU locking, and the page->mem_cgroup stabilization rules.
> > 
> 
> Sorry for not follow you up, did you mean just remove the MEMCG_SWAP configuration
> and keep the feature in default memcg? 

Yes.

> That does can remove lrucare, but PageLRU lock scheme still fails since
> we can not isolate the page during commit_charge, is that right?

No, without lrucare the scheme works. Charges usually do:

page->mem_cgroup = new;
SetPageLRU(page);

And so if you can TestClearPageLRU(), page->mem_cgroup is stable.

lrucare charging is the exception: it changes page->mem_cgroup AFTER
PageLRU has already been set, and even when it CANNOT acquire the
PageLRU lock itself. It violates the rules.

If we make MEMCG_SWAP mandatory, we always have cgroup records for
swapped out pages. That means we can charge all swapin pages
(incl. readahead pages) directly in __read_swap_cache_async(), before
setting PageLRU on the new pages.

Then we can delete lrucare.

And then TestClearPageLRU() guarantees page->mem_cgroup is stable.