Re: [PATCH v3 1/4] mm/zswap: Make shrink_worker writeback cursor per-memcg

[Hao Jia <jiahao.kernel@gmail.com>]

On 2026/6/13 02:15, Yosry Ahmed wrote:
> On Fri, Jun 12, 2026 at 9:40 AM Shakeel Butt <shakeel.butt@linux.dev> wrote:
>>
>> On Thu, Jun 11, 2026 at 05:39:16PM +0000, Yosry Ahmed wrote:
>>> On Tue, Jun 09, 2026 at 11:18:26AM +0800, Hao Jia wrote:
>>>>
>>>>
>>>> On 2026/6/9 02:01, Nhat Pham wrote:
>>>>> On Mon, Jun 8, 2026 at 9:48 AM Yosry Ahmed <yosry@kernel.org> wrote:
>>>>>>
>>>>>>> But OTOH, this does seem like a recipe for inefficient reclaim. We
>>>>>>> might exhaust hotter memory of a cgroup while sparing colder memory of
>>>>>>> another cgroup... But maybe if they're all cold anyway, then who
>>>>>>> cares, and eventually you'll get to the cold stuff of other child?
>>>>>>
>>>>>> Forgot to respond to this part, the unfairness is limited to the batch
>>>>>> size per-invocation, so it should be fine as long as you don't divide
>>>>>> the amount over 100 iterations for some reason. Also yes, all memory
>>>>>> in zswap is cold, the relative coldness is not that important (e.g.
>>>>>> compared to relative coldness during reclaim).
>>>>>
>>>>> Ok then yeah, I think we should shelve per-memcg cursor for the next
>>>>> version. Down the line, if we have more data that unfairness is an
>>>>> issue, we can always fix it. One step at a time :)
>>>>
>>>> Thanks a lot to Yosry, Nhat, and Shakeel for the great suggestions!
>>>>
>>>> Let me summarize what I plan to do in the next version to make sure we are
>>>> on the same page:
>>>>
>>>>   - Drop the per-memcg cursor and keep the root cgroup cursor
>>>> (zswap_next_shrink) logic intact.
>>>>   - Stick to using the zswap_writeback_only key, and change the proactive
>>>> writeback size to use the compressed size.
>>>>   - Consolidate and reuse the logic between shrink_worker() and
>>>> shrink_memcg(). Enable batch writeback in the shrink_worker() path, while
>>>> keeping the writeback behavior in the zswap_store() path unchanged.
>>>>
>>>> Please let me know if I missed or misunderstood anything. Thanks again for
>>>> clearing things up!
>>>
>>> Sorry for the late response, yes I think this makes sense. However, I
>>> have some comment about how this interacts with swap tiering, let me
>>> reply to the other thread.
>>>
>>
>> I think the swap tiers interaction will be figured out over next cycle. However
>> Hao can/should continue to push and we may decide to let it in orthogonal to
>> swap tiers.
> 
> Yeah I think there are a lot of changes we discussed outside of the
> memcg interface, so maybe keep the interface as-is for now, work on a
> new version with the other changes, and we can finalize the interface
> at the end?

Okay, I will split the non-memcg interface parts into a few separate 
patches. These will serve as the preparation work for proactive 
writeback and enable batch writeback in the shrink_worker() path.

However, I will still send the complete patchset using the 
zswap_writeback_only key approach in the next version. This should make 
it easier to review whether the preparation logic is reasonable, and to 
decide whether it should eventually be merged independently of the swap 
tiers.

Thanks,
Hao