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

[Chengming Zhou <zhouchengming@bytedance.com>]

On 2024/1/25 16:42, Yosry Ahmed wrote:
> On Thu, Jan 25, 2024 at 12:30 AM Chengming Zhou
> <zhouchengming@bytedance.com> wrote:
>>
>> On 2024/1/25 15:53, Yosry Ahmed wrote:
>>>> Hello,
>>>>
>>>> I also thought about this problem for some time, maybe something like below
>>>> can be changed to fix it? It's likely I missed something, just some thoughts.
>>>>
>>>> IMHO, the problem is caused by the different way in which we use zswap entry
>>>> in the writeback, that should be much like zswap_load().
>>>>
>>>> The zswap_load() comes in with the folio locked in swap cache, so it has
>>>> stable zswap tree to search and lock... But in writeback case, we don't,
>>>> shrink_memcg_cb() comes in with only a zswap entry with lru list lock held,
>>>> then release lru lock to get tree lock, which maybe freed already.
>>>>
>>>> So we should change here, we read swpentry from entry with lru list lock held,
>>>> then release lru lock, to try to lock corresponding folio in swap cache,
>>>> if we success, the following things is much the same like zswap_load().
>>>> We can get tree lock, to recheck the invalidate race, if no race happened,
>>>> we can make sure the entry is still right and get refcount of it, then
>>>> release the tree lock.
>>>
>>> Hmm I think you may be onto something here. Moving the swap cache
>>> allocation ahead before referencing the tree should give us the same
>>> guarantees as zswap_load() indeed. We can also consolidate the
>>> invalidate race checks (right now we have one in shrink_memcg_cb() and
>>> another one inside zswap_writeback_entry()).
>>
>> Right, if we successfully lock folio in the swap cache, we can get the
>> tree lock and check the invalidate race, only once.
>>
>>>
>>> We will have to be careful about the error handling path to make sure
>>> we delete the folio from the swap cache only after we know the tree
>>> won't be referenced anymore. Anyway, I think this can work.
>>
>> Yes, we can't reference tree if we early return or after unlocking folio,
>> since the reference of zswap entry can't protect the tree.
>>
>>>
>>> On a separate note, I think there is a bug in zswap_writeback_entry()
>>> when we delete a folio from the swap cache. I think we are missing a
>>> folio_unlock() there.
>>
>> Ah, yes, and folio_put().
> 
> Yes. I am preparing a fix.
> 
>>
>>>
>>>>
>>>> The main differences between this writeback with zswap_load() is the handling
>>>> of lru entry and the tree lifetime. The whole zswap_load() function has the
>>>> stable reference of zswap tree, but it's not for shrink_memcg_cb() bottom half
>>>> after __swap_writepage() since we unlock the folio after that. So we can't
>>>> reference the tree after that.
>>>>
>>>> This problem is easy to fix, we can zswap_invalidate_entry(tree, entry) early
>>>> in tree lock, since thereafter writeback can't fail. BTW, I think we should
>>>> also zswap_invalidate_entry() early in zswap_load() and only support the
>>>> zswap_exclusive_loads_enabled mode, but that's another topic.
>>>
>>> zswap_invalidate_entry() actually doesn't seem to be using the tree at all.
>>>
>>>>
>>>> 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.