On Tue, May 30, 2023 at 2:15 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Tue, 30 May 2023 21:02:51 +0000 Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote:
>
> > Commit 71024cb4a0bf ("frontswap: remove frontswap_tmem_exclusive_gets")
> > removed support for exclusive loads from frontswap as it was not used.
> >
> > Bring back exclusive loads support to frontswap by adding an
> > exclusive_loads argument to frontswap_ops. Add support for exclusive
> > loads to zswap behind CONFIG_ZSWAP_EXCLUSIVE_LOADS.
>
> Why is this Kconfigurable? Why not just enable the feature for all
> builds?
I assumed that some users want the current behavior, where reclaiming
clean pages that were once in zswap would be faster. If no one cares,
I can remove the config option and have it always on.
>
> > Refactor zswap entry invalidation in zswap_frontswap_invalidate_page()
> > into zswap_invalidate_entry() to reuse it in zswap_frontswap_load().
> >
> > With exclusive loads, we avoid having two copies of the same page in
> > memory (compressed & uncompressed) after faulting it in from zswap. On
> > the other hand, if the page is to be reclaimed again without being
> > dirtied, it will be re-compressed. Compression is not usually slow, and
> > a page that was just faulted in is less likely to be reclaimed again
> > soon.
> >
> > --- a/mm/Kconfig
> > +++ b/mm/Kconfig
> > @@ -46,6 +46,19 @@ config ZSWAP_DEFAULT_ON
> > The selection made here can be overridden by using the kernel
> > command line 'zswap.enabled=' option.
> >
> > +config ZSWAP_EXCLUSIVE_LOADS
> > + bool "Invalidate zswap entries when pages are loaded"
> > + depends on ZSWAP
> > + help
> > + If selected, when a page is loaded from zswap, the zswap entry is
> > + invalidated at once, as opposed to leaving it in zswap until the
> > + swap entry is freed.
> > +
> > + This avoids having two copies of the same page in memory
> > + (compressed and uncompressed) after faulting in a page from zswap.
> > + The cost is that if the page was never dirtied and needs to be
> > + swapped out again, it will be re-compressed.
>
> So it's a speed-vs-space tradeoff? I'm not sure how users are to
> decide whether they want this. Did we help them as much as possible?
Yes, it is a reclaim speed vs. space tradeoff.
My intuition is that it should be more useful to have this enabled, as
the memory savings should be more important than having reclaim be a
little bit faster in some specific situations. We can make the
configuration on by default if others agree.
I would imagine users would turn this configuration on and observe
memory usage of zswap vs. reclaim speed, and decide based on the
numbers.
>
>
