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

[Andrew Morton]

On Sat, 28 Jun 2025 17:04:31 +0530 Dev Jain <dev.jain at 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?

> 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.
>