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Date:   Mon, 9 May 2022 11:54:49 -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>
Subject: Re: [PATCH v2] mm: don't be stuck to rmap lock on reclaim path
Message-Id: <20220509115449.f48559dd40a5e5ec95b8ead8@linux-foundation.org>
In-Reply-To: <20220509154710.4132957-1-minchan@kernel.org>
References: <20220509154710.4132957-1-minchan@kernel.org>
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On Mon,  9 May 2022 08:47:10 -0700 Minchan Kim <minchan@kernel.org> wrote:

> The rmap locks(i_mmap_rwsem and anon_vma->root->rwsem) could be
> contented under memory pressure if processes keep working on
> their vmas(e.g., fork, mmap, munmap). It makes reclaim path
> stuck. In our real workload traces, we see kswapd is waiting the
> lock for 300ms+(a sec as worst case) and it makes other processes
> entering direct reclaim, which were also stuck on the lock.
> 
> This patch makes LRU aging path try_lock mode like shink_page_list
> so the reclaim context will keep working with next LRU pages
> without being stuck.
> 
> Since this patch introduces a new "contended" field as out-param
> along with try_lock in-param in rmap_walk_control, it's not
> immutable any longer if the try_lock is set so remove const
> keywords on rmap related functions. Since rmap walking is already
> expensive operation, I doubt the const would help sizable benefit(
> And we didn't have it until 5.17).

Some quantitative testing results would be helpful.  Demonstrate
the benefits of the patch?