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

[Huang, Ying]

Yosry Ahmed <yosryahmed@xxxxxxxxxx> writes:

> 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?

In fact, the swap entries to be freed by swapcache_free_entries() is in
a state that can not be freed by other path (including swapoff()).  It's
swap_map value is SWAP_HAS_CACHE, but we can not find folio in
swap_address_space().

To be honest, I don't know whether there are dependencies on sis->lock
in these callbacks.  You need to investigate them one by one.

--
Best Regards,
Huang, Ying