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Date:   Thu, 5 Aug 2021 23:01:27 -0700 (PDT)
From:   Hugh Dickins <hughd@google.com>
To:     Yang Shi <shy828301@gmail.com>
cc:     Hugh Dickins <hughd@google.com>,
        Matthew Wilcox <willy@infradead.org>,
        "Huang, Ying" <ying.huang@intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        David Hildenbrand <david@redhat.com>,
        Linux MM <linux-mm@kvack.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Miaohe Lin <linmiaohe@huawei.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@suse.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Minchan Kim <minchan@kernel.org>
Subject: Re: [PATCH] mm,shmem: Fix a typo in shmem_swapin_page()
In-Reply-To: <CAHbLzkrShKORwU-Zrf5_5c6_o_iVwoXRTT+7ABZALLj7p0w-Ug@mail.gmail.com>
Message-ID: <d1c768c-8b6-6a9d-ddde-39ac51286927@google.com>
References: <20210723080000.93953-1-ying.huang@intel.com> <24187e5e-069-9f3f-cefe-39ac70783753@google.com> <YPs6cQo7iG1JcOn8@casper.infradead.org> <8735rr54i9.fsf@yhuang6-desk2.ccr.corp.intel.com> <YQkwBdpWTPjv4xIx@casper.infradead.org>
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On Thu, 5 Aug 2021, Yang Shi wrote:
> On Tue, Aug 3, 2021 at 10:34 PM Hugh Dickins <hughd@google.com> wrote:
> >
> > I've never seen the swapoff race claimed by Miaohe, and don't expect to;
> > but he's probably right, given the current code.  I just dislike adding
> > unnecessary complexity, and siting it in the wrong place (mm/shmem.c).
> >
> > Yang, is it possible that 5.1 commit 8fd2e0b505d1 ("mm: swap: check if
> > swap backing device is congested or not") was actually developed and
> > measured on 4.1 or earlier, which still had blk_set_queue_congested()?
> 
> I forgot the exact version, but definitely not 4.1 or earlier. Maybe
> 4.19 or earlier. I'm not familiar with how block layer detect
> congestion, if the logic was changed, hence the optimization doesn't
> stand anymore nowadays, I'm totally fine to remove it.

You drove me back to look more closely.  blk_set_queue_congested()
vanished from include/linux/blkdev.h in 4.2, but blk_set_congested()
appeared then in block/blk-core.c to replace it.  blk_set_congested()
vanished (along with all references to "congested" in blk-core.c) in
5.0, then your commit (most probably tested on 4.19) went into 5.1 -
just after it had become redundant!

Thanks, yes, let's revert that and Miaohe's and Huang's, later on.

Hugh