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

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

On 30/06/25 4:47 pm, Lorenzo Stoakes wrote:
> On Sat, Jun 28, 2025 at 05:04:31PM +0530, Dev Jain 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.
> Should also be tested on x86-64 not only build tested :)
>
> You are still not really giving details here, so same comment as your mremap()
> series, please explain why you're doing this, what for, what benefits you expect
> to achieve, where etc.
>
> E.g. 'this is deisgned to optimise mTHP cases on arm64, we expect to see
> benefits on amd64 also and for intel there should be no impact'.

Okay.

>
> It's probably also worth actually going and checking to make sure that this is
> the case re: other arches. See below on that...
>
>> 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
>>
>> 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.
> This is nice, though order-0 is probably going to be your bread and butter no?
>
> Having said that, mprotect() is not a hot path, this delta is small enough to
> quite possibly just be noise, and personally I'm not all that bothered.

It is only the vm_normal_folio() + folio_test_large() overhead. Trying to avoid
this by the horrible maybe_contiguous_pte_pfns() I introduced somewhere else
is not worth it : )

>
> But let's run this same test on x86-64 too please and get some before/after
> numbers just to confirm no major impact.
>
> Thanks for including code.
>
>> Here is the test program:
>>
>>   #define _GNU_SOURCE
>>   #include <sys/mman.h>
>>   #include <stdlib.h>
>>   #include <string.h>
>>   #include <stdio.h>
>>   #include <unistd.h>
>>
>>   #define SIZE (1024*1024*1024)
>>
>> unsigned long pmdsize = (1UL << 21);
>> unsigned long pagesize = (1UL << 12);
>>
>> static void pte_map_thps(char *mem, size_t size)
>> {
>> 	size_t offs;
>> 	int ret = 0;
>>
>>
>> 	/* PTE-map each THP by temporarily splitting the VMAs. */
>> 	for (offs = 0; offs < size; offs += pmdsize) {
>> 		ret |= madvise(mem + offs, pagesize, MADV_DONTFORK);
>> 		ret |= madvise(mem + offs, pagesize, MADV_DOFORK);
>> 	}
>>
>> 	if (ret) {
>> 		fprintf(stderr, "ERROR: mprotect() failed\n");
>> 		exit(1);
>> 	}
>> }
>>
>> int main(int argc, char *argv[])
>> {
>> 	char *p;
>>          int ret = 0;
>> 	p = mmap((1UL << 30), SIZE, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
>> 	if (p != (1UL << 30)) {
>> 		perror("mmap");
>> 		return 1;
>> 	}
>>
>>
>>
>> 	memset(p, 0, SIZE);
>> 	if (madvise(p, SIZE, MADV_NOHUGEPAGE))
>> 		perror("madvise");
>> 	explicit_bzero(p, SIZE);
>> 	pte_map_thps(p, SIZE);
>>
>> 	for (int loops = 0; loops < 40; loops++) {
>> 		if (mprotect(p, SIZE, PROT_READ))
>> 			perror("mprotect"), exit(1);
>> 		if (mprotect(p, SIZE, PROT_READ|PROT_WRITE))
>> 			perror("mprotect"), exit(1);
>> 		explicit_bzero(p, SIZE);
>> 	}
>> }
>>
>> ---
>> The patchset is rebased onto Saturday's mm-new.
>>
>> v3->v4:
>>   - Refactor skipping logic into a new function, edit patch 1 subject
>>     to highlight it is only for MM_CP_PROT_NUMA case (David H)
>>   - Refactor the optimization logic, add more documentation to the generic
>>     batched functions, do not add clear_flush_ptes, squash patch 4
>>     and 5 (Ryan)
>>
>> v2->v3:
>>   - Add comments for the new APIs (Ryan, Lorenzo)
>>   - Instead of refactoring, use a "skip_batch" label
>>   - Move arm64 patches at the end (Ryan)
>>   - In can_change_pte_writable(), check AnonExclusive page-by-page (David H)
>>   - Resolve implicit declaration; tested build on x86 (Lance Yang)
>>
>> v1->v2:
>>   - Rebase onto mm-unstable (6ebffe676fcf: util_macros.h: make the header more resilient)
>>   - Abridge the anon-exclusive condition (Lance Yang)
>>
>> Dev Jain (4):
>>    mm: Optimize mprotect() for MM_CP_PROT_NUMA by batch-skipping PTEs
>>    mm: Add batched versions of ptep_modify_prot_start/commit
>>    mm: Optimize mprotect() by PTE-batching
>>    arm64: Add batched versions of ptep_modify_prot_start/commit
>>
>>   arch/arm64/include/asm/pgtable.h |  10 ++
>>   arch/arm64/mm/mmu.c              |  28 +++-
>>   include/linux/pgtable.h          |  83 +++++++++-
>>   mm/mprotect.c                    | 269 +++++++++++++++++++++++--------
>>   4 files changed, 315 insertions(+), 75 deletions(-)
>>
>> --
>> 2.30.2
>>