Re: [PATCH v5 0/7] Optimize mprotect() for large folios

[Dev Jain]

On 18/07/25 2:32 pm, Dev Jain wrote:
> Use folio_pte_batch() to optimize change_pte_range(). On arm64, if the ptes
> are painted with the contig bit, then ptep_get() will iterate through all
> 16 entries to collect a/d bits. Hence this optimization will result in
> a 16x reduction in the number of ptep_get() calls. Next,
> ptep_modify_prot_start() will eventually call contpte_try_unfold() on
> every contig block, thus flushing the TLB for the complete large folio
> range. Instead, use get_and_clear_full_ptes() so as to elide TLBIs on
> each contig block, and only do them on the starting and ending
> contig block.
>
> For split folios, there will be no pte batching; the batch size returned
> by folio_pte_batch() will be 1. For pagetable split folios, the ptes will
> still point to the same large folio; for arm64, this results in the
> optimization described above, and for other arches, a minor improvement
> is expected due to a reduction in the number of function calls.
>
> mm-selftests pass on arm64. I have some failing tests on my x86 VM already;
> no new tests fail as a result of this patchset.
>
> 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: 2.1 seconds   T2: 2 seconds   T3: 1 second
>
> After the patchset:
> T1: 0.65 seconds   T2: 0.7 seconds   T3: 1.1 seconds

For the note: the numbers are different from the previous versions.
I must have run the test for more number of iterations and then
pasted the test program here for 40 iterations, that's why the mismatch.