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Date:   Wed, 11 May 2022 19:05:23 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Minchan Kim <minchan@kernel.org>
Cc:     LKML <linux-kernel@vger.kernel.org>, linux-mm <linux-mm@kvack.org>,
        Suren Baghdasaryan <surenb@google.com>,
        Michal Hocko <mhocko@suse.com>,
        John Dias <joaodias@google.com>,
        Tim Murray <timmurray@google.com>,
        Matthew Wilcox <willy@infradead.org>,
        Vladimir Davydov <vdavydov.dev@gmail.com>,
        Martin Liu <liumartin@google.com>,
        Johannes Weiner <hannes@cmpxchg.org>
Subject: Re: [PATCH v4] mm: don't be stuck to rmap lock on reclaim path
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On Wed, 11 May 2022 15:57:09 -0700 Minchan Kim <minchan@kernel.org> wrote:

> > 
> > Could we burn much CPU time pointlessly churning though the LRU?  Could
> > it mess up aging decisions enough to be performance-affecting in any
> > workload?
> 
> Yes, correct. However, we are already churning LRUs by several
> ways. For example, isolate and putback from LRU list for page
> migration from several sources(typical example is compaction)
> and trylock_page and sc->gfp_mask not allowing page to be
> reclaimed in shrink_page_list.

Well.  "we're already doing a risky thing so it's OK to do more of that
thing"?

> > 
> > Something else?
> 
> One thing I am worry about was the granularity of the churning.
> Example above was page granuarity churning so might be execuse
> but this one is address space's churning, especically for file LRU
> (i_mmap_rwsem) which might cause too many rotating and live-lock
> in the end(keey rotating in small LRU with heavy memory pressure).
> 
> If it could be a problem, maybe we use sc->priority to stop
> the skipping on a certain level of memory pressure.
> 
> Any thought? Do we really need it?

Are we able to think of a test which might demonstrate any worst case? 
Whip that up and see what the numbers say?

It's a bit of a drag, but if we don't do it, our users surely will ;)