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

[Nhat Pham]

On Tue, Sep 24, 2024 at 1:51 PM Sridhar, Kanchana P
<kanchana.p.sridhar@xxxxxxxxx> 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?

>
> 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