Re: [PATCH 2/2] mm: zswap: remove unnecessary tree cleanups in zswap_swapoff()

[Chengming Zhou]

On 2024/1/25 17:03, Yosry Ahmed wrote:
>>>>>> The second difference is the handling of lru entry, which is easy that we
>>>>>> just zswap_lru_del() in tree lock.
>>>>>
>>>>> Why do we need zswap_lru_del() at all? We should have already isolated
>>>>> the entry at that point IIUC.
>>>>
>>>> I was thinking how to handle the "zswap_lru_putback()" if not writeback,
>>>> in which case we can't use the entry actually since we haven't got reference
>>>> of it. So we can don't isolate at the entry, and only zswap_lru_del() when
>>>> we are going to writeback actually.
>>>
>>> Why not just call zswap_lru_putback() before we unlock the folio?
>>
>> When early return because __read_swap_cache_async() return NULL or !folio_was_allocated,
>> we don't have a locked folio yet. The entry maybe invalidated and freed concurrently.
> 
> Oh, that path, right.
> 
> If we don't isolate the entry straightaway, concurrent reclaimers will
> see the same entry, call __read_swap_cache_async(), find the folio
> already in the swapcache and stop shrinking. This is because usually
> this means we are racing with swapin and hitting the warmer part of
> the zswap LRU.
> 
> I am not sure if this would matter in practice, maybe Nhat knows
> better. Perhaps we can rotate the entry in the LRU before calling
> __read_swap_cache_async() to minimize the chances of such a race? Or
> we can serialize the calls to __read_swap_cache_async() but this may
> be an overkill.

Also, not sure, rotate the entry maybe good IMHO since we will zswap_lru_del()
once we checked the invalidate race.