Re: [PATCH V5] x86/mm: Tracking linear mapping split events

[Dave Hansen]

On 1/28/21 8:33 AM, Zi Yan wrote:
>> One of the many lasting (as we don't coalesce back) sources for
>> huge page splits is tracing as the granular page
>> attribute/permission changes would force the kernel to split code
>> segments mapped to huge pages to smaller ones thereby increasing
>> the probability of TLB miss/reload even after tracing has been
>> stopped.
> It is interesting to see this statement saying splitting kernel
> direct mappings causes performance loss, when Zhengjun (cc’d) from
> Intel recently posted a kernel direct mapping performance report[1]
> saying 1GB mappings are good but not much better than 2MB and 4KB
> mappings.

No, that's not what the report said.

*Overall*, there is no clear winner between 4k, 2M and 1G.  In other
words, no one page size is best for *ALL* workloads.

There were *ABSOLUTELY* individual workloads in those tests that saw
significant deltas between the direct map sizes.  There are also
real-world workloads that feel the impact here.