Re: [PATCH v2] mm: shmem: skip swapcache for swapin of synchronous swap device

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 2025/1/6 12:07, Matthew Wilcox wrote:
> On Mon, Jan 06, 2025 at 11:46:04AM +0800, Baolin Wang wrote:
>> On 2025/1/2 21:10, Matthew Wilcox wrote:
>>> On Thu, Jan 02, 2025 at 04:40:17PM +0800, Baolin Wang wrote:
>>>> With fast swap devices (such as zram), swapin latency is crucial to applications.
>>>> For shmem swapin, similar to anonymous memory swapin, we can skip the swapcache
>>>> operation to improve swapin latency.
>>>
>>> OK, but now we have more complexity.  Why can't we always skip the
>>> swapcache on swapin?
>>
>> Skipping swapcache is used to swap-in shmem large folios, avoiding the large
>> folios being split. Meanwhile, since the IO latency of syncing swap devices
>> is relatively small, it won't cause the IO latency amplification issue.
>>
>> But for async swap devices, if we swap-in the large folio one-time, I am
>> afraid the IO latency can be amplified. And I remember we still haven't
>> reached an agreement here[1], so let's step by step and start with the sync
>> swap devices first.
> 
> Regardless of whether we choose to swap-in an order-0 or a large folio,
> my point is that we should always do it to the pagecache rather than the
> swap cache.

IMO, this would miss the swap readahead algorithm in the swap case, 
which can benefit the order-0 swap-in. We need more work to ensure that 
skipping swapcache is helpful for all cases, which is why I'm starting 
with sync swap devices first.