Re: [PATCH v7 6/8] mm: zswap: Support mTHP swapout in zswap_store().

[Yosry Ahmed <yosryahmed@google.com>]

On Tue, Sep 24, 2024 at 2:08 PM Nhat Pham <nphamcs@gmail.com> wrote:
>
> On Tue, Sep 24, 2024 at 1:51 PM Sridhar, Kanchana P
> <kanchana.p.sridhar@intel.com> wrote:
> >
> >
> > This is an excellent point. Thanks Nhat for catching this! I can see two
> > options to solving this:
> >
> > Option 1: If zswap_mthp_enabled is "false", delete all stored offsets
> > for the mTHP in zswap before exiting. This could race with writeback
> > (either one or more subpages could be written back before zswap_store
> > acquires the tree lock), however, I don't think it will cause data inconsistencies.
> > Any offsets for subpages not written back will be deleted from zswap,
> > zswap_store() will return false, and the backing swap device's subsequent
> > swapout will over-write the zswap write-back data. Could anything go wrong
> > with this?
>
> I think this should be safe, albeit a bit awkward.
>
> At this point (zswap_store()), we should have the folio added to to
> swap cache, and locked. All the associated swap entries will point to
> this same (large) folio.
>
> Any concurrent zswap writeback attempt, even on a tail page, should
> get that folio when it calls __read_swap_cache_async(), and with
> page_allocated == false, and should short circuit.
>
> So I don't think we will race with zswap_writeback().
>
> Yosry, Chengming, Johannes, any thoughts?

Why can't we just handle it the same way as we handle zswap
disablement? If it is disabled, we invalidate any old entries for the
offsets and return false to swapout to disk.

Taking a step back, why do we need the runtime knob and config option?
Are there cases where we think zswapout of mTHPs will perform badly,
or is it just due to lack of confidence in the feature?

>
> >
> > Option 2: Only provide a build config option,
> > CONFIG_ZSWAP_STORE_THP_DEFAULT_ON, that cannot be dynamically changed.
>
> This can be a last resort thing, if the above doesn't work. Not the
> end of the world, but not ideal :)
>
> >
> > Would appreciate suggestions on these, and other potential solutions.
> >
> > Thanks,
> > Kanchana