Re: [PATCH 2/5] mm/zswap: change dstmem size to one page

[Chengming Zhou]

On 2023/12/14 21:37, Yosry Ahmed wrote:
> On Thu, Dec 14, 2023 at 5:33 AM Chengming Zhou
> <zhouchengming@xxxxxxxxxxxxx> wrote:
>>
>> On 2023/12/14 08:18, Nhat Pham wrote:
>>> On Wed, Dec 13, 2023 at 3:34 PM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote:
>>>>
>>>> On Tue, Dec 12, 2023 at 8:18 PM Chengming Zhou
>>>> <zhouchengming@xxxxxxxxxxxxx> wrote:
>>>>>
>>>>> Change the dstmem size from 2 * PAGE_SIZE to only one page since
>>>>> we only need at most one page when compress, and the "dlen" is also
>>>>> PAGE_SIZE in acomp_request_set_params(). If the output size > PAGE_SIZE
>>>>> we don't wanna store the output in zswap anyway.
>>>>>
>>>>> So change it to one page, and delete the stale comment.
>>>>
>>>> I couldn't find the history of why we needed 2 * PAGE_SIZE, it would
>>>> be nice if someone has the context, perhaps one of the maintainers.
>>>
>>> It'd be very nice indeed.
>>>
>>>>
>>>> One potential reason is that we used to store a zswap header
>>>> containing the swap entry in the compressed page for writeback
>>>> purposes, but we don't do that anymore. Maybe we wanted to be able to
>>>> handle the case where an incompressible page would exceed PAGE_SIZE
>>>> because of that?
>>>
>>> It could be hmm. I didn't study the old zswap architecture too much,
>>> but it has been 2 * PAGE_SIZE since the time zswap was first merged
>>> last I checked.
>>> I'm not 100% comfortable ACK-ing the undoing of something that looks
>>> so intentional, but FTR, AFAICT, this looks correct to me.
>>
>> Right, there is no any history about the reason why we needed 2 pages.
>> But obviously only one page is needed from the current code and no any
>> problem found in the kernel build stress testing.
> 
> Could you try manually stressing the compression with data that
> doesn't compress at all (i.e. dlen == PAGE_SIZE)? I want to make sure
> that this case is specifically handled. I think using data from
> /dev/random will do that but please double check that dlen ==
> PAGE_SIZE.

I just did the same kernel build testing, indeed there are a few cases
that output dlen == PAGE_SIZE.

bpftrace -e 'k:zpool_malloc {@[(uint32)arg1==4096]=count()}'

@[1]: 2
@[0]: 12011430