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 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.

--
Best Regards,
Huang, Ying