linux-kernel - [PATCH v3 0/5] Optimize mprotect() for large folios

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Message-Id: <20250519074824.42909-1-dev.jain@arm.com>
Date: Mon, 19 May 2025 13:18:19 +0530
From: Dev Jain <dev.jain@....com>
To: akpm@...ux-foundation.org
Cc: ryan.roberts@....com,
	david@...hat.com,
	willy@...radead.org,
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	Dev Jain <dev.jain@....com>
Subject: [PATCH v3 0/5] Optimize mprotect() for large folios

This patchset optimizes the mprotect() system call for large folios
by PTE-batching.

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.

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 mm-unstable (9ead4336d7c07f085def6ab372245640a22af5bd).

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 (5):
  mm: Optimize mprotect() by batch-skipping PTEs
  mm: Add batched versions of ptep_modify_prot_start/commit
  mm: Optimize mprotect() by PTE batching
  arm64: Add batched version of ptep_modify_prot_start
  arm64: Add batched version of ptep_modify_prot_commit

 arch/arm64/include/asm/pgtable.h |  10 ++
 arch/arm64/mm/mmu.c              |  21 ++++-
 include/linux/mm.h               |   7 +-
 include/linux/pgtable.h          |  79 ++++++++++++++++
 mm/mprotect.c                    | 154 +++++++++++++++++++++++++------
 mm/pgtable-generic.c             |  16 +++-
 6 files changed, 246 insertions(+), 41 deletions(-)

-- 
2.30.2