Re: [PATCH v4 0/4] Optimize mprotect() for large folios

[Dev Jain <dev.jain@arm.com>]

On 30/06/25 4:35 am, Andrew Morton wrote:
> On Sat, 28 Jun 2025 17:04:31 +0530 Dev Jain <dev.jain@arm.com> wrote:
>
>> This patchset optimizes the mprotect() system call for large folios
>> by PTE-batching. No issues were observed with mm-selftests, build
>> tested on x86_64.
> um what.  Seems to claim that "selftests still compiles after I messed
> with stuff", which isn't very impressive ;)  Please clarify?

Sorry I mean to say that the mm-selftests pass.

>
>> We use the following test cases to measure performance, mprotect()'ing
>> the mapped memory to read-only then read-write 40 times:
>>
>> Test case 1: Mapping 1G of memory, touching it to get PMD-THPs, then
>> pte-mapping those THPs
>> Test case 2: Mapping 1G of memory with 64K mTHPs
>> Test case 3: Mapping 1G of memory with 4K pages
>>
>> Average execution time on arm64, Apple M3:
>> Before the patchset:
>> T1: 7.9 seconds   T2: 7.9 seconds   T3: 4.2 seconds
>>
>> After the patchset:
>> T1: 2.1 seconds   T2: 2.2 seconds   T3: 4.3 seconds
> Well that's tasty.
>
>> Observing T1/T2 and T3 before the patchset, we also remove the regression
>> introduced by ptep_get() on a contpte block. And, for large folios we get
>> an almost 74% performance improvement, albeit the trade-off being a slight
>> degradation in the small folio case.
>>