linux-kernel - Re: [PATCH v4] mm: Enable suspend-only swap spaces

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Date:   Tue, 27 Jul 2021 14:18:32 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Evan Green <evgreen@...omium.org>
Cc:     David Hildenbrand <david@...hat.com>,
        Michal Hocko <mhocko@...e.com>, Pavel Machek <pavel@....cz>,
        linux-api@...r.kernel.org, Alex Shi <alexs@...nel.org>,
        Alistair Popple <apopple@...dia.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Minchan Kim <minchan@...nel.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org
Subject: Re: [PATCH v4] mm: Enable suspend-only swap spaces

On Tue, 27 Jul 2021 09:31:33 -0700 Evan Green <evgreen@...omium.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?