Re: [PATCH v9 4/8] mm: add a template-based fast path for zone-device page init

["Li Zhe" <lizhe.67@bytedance.com>]

On 8/3/26 4:39 PM, Muchun Song wrote:
>
>> On Aug 3, 2026, at 15:09, Li Zhe <lizhe.67@bytedance.com> wrote:
>>
>> memmap_init_zone_device() repeats nearly identical head-page
>> initialization for each PFN. Prepare one reusable ZONE_DEVICE head-page
>> template through the existing slow path, refresh the PFN-dependent
>> fields in that template before each copy, and memcpy it into each
>> destination page.
>>
>> Use the template path unconditionally, as suggested by Muchun. The
>> page_ref_set tracepoint is primarily a debugging aid, while this code is
>> still initializing struct pages before they are handed out. From the
>> perspective of users of those pages, the initialization-time refcount
>> transitions are not part of the observable page lifetime.
>>
>> This means page_ref_set will no longer observe every initialization-time
>> refcount assignment for copied ZONE_DEVICE head pages. The impact is
>> controlled because the final initialized struct page state is unchanged,
>> and keeping a separate non-template path only for this local tracepoint
>> observability would add complexity to the common path.
>>
>> This patch accelerates head-page initialization. The pfns_per_compound
>> == 1 case gets the full benefit here, compound tails are handled in the
>> next patch.
>>
>> Tested in a VM with a 100 GB fsdax namespace device configured with
>> map=dev on Intel Ice Lake server. This test exercises the nd_pmem rebind
>> path (pfns_per_compound == 1).
>>
>> Test procedure:
>> Rebind the nd_pmem driver 30 times and collect the memmap initialization
>> time from the pr_debug() output of memmap_init_zone_device().
>>
>> Base(v7.2-rc1):
>>   Average of rebinds for nd_pmem driver: 244.28 ms
>>
>> With this patch and its prerequisites applied:
>>   Average of rebinds for nd_pmem driver: 215.55 ms
>>
>> This reduces the average memmap initialization time measured during rebind
>> from 244.28 ms to 215.55 ms, or about 11%.
>>
>> Suggested-by: Muchun Song <muchun.song@linux.dev>
> I personally feel you might have used the wrong trailer here, because it
> implies that the overall design is based on someone's suggestion. But for me,
> I think what you actually meant is that in a previous version, I made a
> suggestion about simplifying the code, rather than dictating the overall
> direction of this entire patch series. So I feel I shouldn't be credited here.
> This information is already enough just appearing in the change log.
>
> Muchun,
> Thanks.


Thanks.

I will drop it and keep the simplification note only in the changelog.

Thanks,
Zhe