Re: [PATCH] mm:zswap: fix zswap entry reclamation failure in two scenarios

[Yosry Ahmed]

On Sun, Nov 19, 2023 at 7:20 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
>
> Chris Li <chriscli@xxxxxxxxxx> writes:
>
> > On Thu, Nov 16, 2023 at 12:19 PM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote:
> >>
> >> Not bypassing the swap slot cache, just make the callbacks to
> >> invalidate the zswap entry, do memg uncharging, etc when the slot is
> >> no longer used and is entering the swap slot cache (i.e. when
> >> free_swap_slot() is called), instead of when draining the swap slot
> >> cache (i.e. when swap_range_free() is called). For all parts of MM
> >> outside of swap, the swap entry is freed when free_swap_slot() is
> >> called. We don't free it immediately because of caching, but this
> >> should be transparent to other parts of MM (e.g. zswap, memcg, etc).
> >
> > That will cancel the batching effect on the swap slot free, making the
> > common case for  swapping  faults take longer to complete, righ?
> > If I recall correctly, the uncharge is the expensive part of the swap
> > slot free operation.
> > I just want to figure out what we are trading off against. This is not
> > one side wins all situations.
>
> Per my understanding, we don't batch memcg uncharging in
> swap_entry_free() now.  Although it's possible and may improve
> performance.

Yes. It actually causes a long tail in swapin fault latency as Chris
discovered in our prod. I am wondering if doing the memcg uncharging
outside the slots cache will actually amortize the cost instead.

Regardless of memcg charging, which is more complicated, I think we
should at least move the call to zswap_invalidate() before the slots
cache. I would prefer that we move everything non-swapfile specific
outside the slots cache layer (zswap_invalidate(),
arch_swap_invalidate_page(),  clear_shadow_from_swap_cache(),
mem_cgroup_uncharge_swap(), ..).  However, if some of those are
controversial, we can move some of them for now.

When draining free swap slots from the cache, swap_range_free() is
called with nr_entries == 1 anyway, so I can't see how any batching is
going on. If anything it should help amortize the cost.

>
> --
> Best Regards,
> Huang, Ying