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

[Yosry Ahmed]

On Mon, Nov 20, 2023 at 5:55 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
>
> Yosry Ahmed <yosryahmed@xxxxxxxxxx> writes:
>
> > On Mon, Nov 20, 2023 at 4:57 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
> >>
> >> Yosry Ahmed <yosryahmed@xxxxxxxxxx> writes:
> >>
> >> > 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.
> >>
> >> That makes sense for me.
> >>
> >> > 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.
> >>
> >> In swapcache_free_entries(), the sis->lock will be held to free multiple
> >> swap slots via swap_info_get_cont() if possible.  This can reduce
> >> sis->lock contention.
> >
> > Ah yes that's a good point. Since most of these callbacks don't
> > actually access sis, but use the swap entry value itself, I am
> > guessing the reason we need to hold the lock for all these callbacks
> > is to prevent swapoff and swapon reusing the same swap entry on a
> > different swap device, right?
>
> In,
>
> swapcache_free_entries()
>   swap_entry_free()
>     swap_range_free()
>
> Quite some sis fields will be accessed.

I wasn't referring to this code. I was what's preventing us from
moving the callbacks I mentioned outside the lock (zswap_invalidate(),
arch_swap_invalidate_page(),  clear_shadow_from_swap_cache(),
mem_cgroup_uncharge_swap(), ..). I think most or all of them don't
really access sis, but perhaps they need the lock to ensure the swap
entry value does not get reused?

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