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

[Andrew Morton]

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