Re: [PATCH v4 2/6] mm/zswap: reuse dstmem when decompress

[Chengming Zhou]

On 2023/12/28 16:03, Barry Song wrote:
> On Wed, Dec 27, 2023 at 7:32 PM Chengming Zhou
> <zhouchengming@xxxxxxxxxxxxx> wrote:
>>
>> On 2023/12/27 09:24, Barry Song wrote:
>>> On Wed, Dec 27, 2023 at 4:56 AM Chengming Zhou
>>> <zhouchengming@xxxxxxxxxxxxx> wrote:
>>>>
>>>> In the !zpool_can_sleep_mapped() case such as zsmalloc, we need to first
>>>> copy the entry->handle memory to a temporary memory, which is allocated
>>>> using kmalloc.
>>>>
>>>> Obviously we can reuse the per-compressor dstmem to avoid allocating
>>>> every time, since it's percpu-compressor and protected in percpu mutex.
>>>
>>> what is the benefit of this since we are actually increasing lock contention
>>> by reusing this buffer between multiple compression and decompression
>>> threads?
>>
>> This mutex is already reused in all compress/decompress paths even before
>> the reuse optimization. I think the best way maybe to use separate crypto_acomp
>> for compression and decompression.
>>
>> Do you think the lock contention will be increased because we now put zpool_map_handle()
>> and memcpy() in the lock section? Actually, we can move zpool_map_handle() before
>> the lock section if needed, but that memcpy() should be protected in lock section.
>>
>>>
>>> this mainly affects zsmalloc which can't sleep? do we have performance
>>> data?
>>
>> Right, last time when test I remembered there is very minor performance difference.
>> The main benefit here is to simply the code much and delete one failure case.
> 
> ok.
> 
> For the majority of hardware, people are using CPU-based
> compression/decompression,
> there is no chance they will sleep. Thus, all
> compression/decompression can be done
> in a zpool_map section, there is *NO* need to copy at all! Only for

Yes, very good for zsmalloc.

> those hardware which
> can provide a HW-accelerator to offload CPU, crypto will actually wait
> for completion by
> 
> static inline int crypto_wait_req(int err, struct crypto_wait *wait)
> {
>         switch (err) {
>         case -EINPROGRESS:
>         case -EBUSY:
>                 wait_for_completion(&wait->completion);
>                 reinit_completion(&wait->completion);
>                 err = wait->err;
>                 break;
>         }
> 
>         return err;
> }
> 
> for CPU-based alg, we have completed the compr/decompr within
> crypto_acomp_decompress()
> synchronously. they won't return EINPROGRESS, EBUSY.

Ok, this is useful to know.

> 
> The problem is that crypto_acomp won't expose this information to its
> users. if it does,
> we can use this info, we will totally avoid the code of copying
> zsmalloc's data to a tmp
> buffer for the most majority users of zswap.

Agree, I think it's worthwhile to export, so zsmalloc users don't need to
prepare the temporary buffer and copy in the majority case.

Thanks!

> 
> But I am not sure if we can find a way to convince Herbert(+To)  :-)
>