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Date:   Tue, 27 Jul 2021 14:18:32 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Evan Green <evgreen@chromium.org>
Cc:     David Hildenbrand <david@redhat.com>,
        Michal Hocko <mhocko@suse.com>, Pavel Machek <pavel@ucw.cz>,
        linux-api@vger.kernel.org, Alex Shi <alexs@kernel.org>,
        Alistair Popple <apopple@nvidia.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        "Matthew Wilcox (Oracle)" <willy@infradead.org>,
        Miaohe Lin <linmiaohe@huawei.com>,
        Minchan Kim <minchan@kernel.org>,
        Suren Baghdasaryan <surenb@google.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        LKML <linux-kernel@vger.kernel.org>, linux-mm@kvack.org
Subject: Re: [PATCH v4] mm: Enable suspend-only swap spaces
Message-Id: <20210727141832.86695e7181eb10c6e8fd0191@linux-foundation.org>
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References: <20210726171106.v4.1.I09866d90c6de14f21223a03e9e6a31f8a02ecbaf@changeid>
        <d6668437-5c3b-2dff-bb95-4e3132d13711@redhat.com>
        <6ff28cfe-1107-347b-0327-ad36e256141b@redhat.com>
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On Tue, 27 Jul 2021 09:31:33 -0700 Evan Green <evgreen@chromium.org> wrote:

> > Pavel just mentioned uswsusp, and I wonder if it would be a possible
> > alternative to this patch.
> 
> I think you're right that it would be possible to isolate the
> hibernate image with uswsusp if you avoid using the SNAPSHOT_*SWAP*
> ioctls. But I'd expect performance to suffer noticeably, since now
> every page is making a round trip out to usermode and back. I'd still
> very much use the HIBERNATE_ONLY flag if it were accepted, I think
> there's value to it.

The uswsusp option makes your patch a performance optimization rather
than a feature-add.  And we do like to see quantitative testing results
when considering a performance optimization.  Especially when the
performance optimization is a bit icky, putting special-case testing
all over the place, maintenance cost, additional testing effort, etc.

I do think that diligence demands that we quantify the difference.  Is
this a thing you can help with?