[PATCH 1/2] mm: Enable khugepaged to operate on non-writable VMAs

Dev Jain
2025-09-03T05:46:52Z

Currently khugepaged does not collapse a region which does not have a
single writable page. This is wasteful since non-writable VMAs mapped by
the application won't benefit from THP collapse. Therefore, remove this
restriction and allow khugepaged to collapse a VMA with arbitrary
protections.

Along with this, currently MADV_COLLAPSE does not perform a collapse on a
non-writable VMA, and this restriction is nowhere to be found on the
manpage - the restriction itself sounds wrong to me since the user knows
the protection of the memory it has mapped, so collapsing read-only
memory via madvise() should be a choice of the user which shouldn't
be overriden by the kernel.

On an arm64 machine, an average of 5% improvement is seen on some mmtests
benchmarks, particularly hackbench, with a maximum improvement of 12%.

Signed-off-by: Dev Jain <dev.jain@arm.com>
---
RFC->v1:
Drop writable references from tracepoints

RFC:
https://lore.kernel.org/all/20250901074817.73012-1-dev.jain@arm.com/

I can see performance improvements on mmtests run on an arm64 machine
comparing with 6.17-rc2. (I) denotes statistically significant improvement,
(R) denotes statistically significant regression (Please ignore the
numbers in the middle column):

+------------------------------------+----------------------------------------------------------+-----------------------+--------------------------+
| mmtests/hackbench | process-pipes-1 (seconds) | 0.145 | -0.06% |
| | process-pipes-4 (seconds) | 0.4335 | -0.27% |
| | process-pipes-7 (seconds) | 0.823 | (I) -12.13% |
| | process-pipes-12 (seconds) | 1.3538333333333334 | (I) -5.32% |
| | process-pipes-21 (seconds) | 1.8971666666666664 | (I) -2.87% |
| | process-pipes-30 (seconds) | 2.5023333333333335 | (I) -3.39% |
| | process-pipes-48 (seconds) | 3.4305 | (I) -5.65% |
| | process-pipes-79 (seconds) | 4.245833333333334 | (I) -6.74% |
| | process-pipes-110 (seconds) | 5.114833333333333 | (I) -6.26% |
| | process-pipes-141 (seconds) | 6.1885 | (I) -4.99% |
| | process-pipes-172 (seconds) | 7.231833333333334 | (I) -4.45% |
| | process-pipes-203 (seconds) | 8.393166666666668 | (I) -3.65% |
| | process-pipes-234 (seconds) | 9.487499999999999 | (I) -3.45% |
| | process-pipes-256 (seconds) | 10.316166666666666 | (I) -3.47% |
| | process-sockets-1 (seconds) | 0.289 | 2.13% |
| | process-sockets-4 (seconds) | 0.7596666666666666 | 1.02% |
| | process-sockets-7 (seconds) | 1.1663333333333334 | -0.26% |
| | process-sockets-12 (seconds) | 1.8641666666666665 | -1.24% |
| | process-sockets-21 (seconds) | 3.0773333333333333 | 0.01% |
| | process-sockets-30 (seconds) | 4.2405 | -0.15% |
| | process-sockets-48 (seconds) | 6.459666666666666 | 0.15% |
| | process-sockets-79 (seconds) | 10.156833333333333 | 1.45% |
| | process-sockets-110 (seconds) | 14.317833333333333 | -1.64% |
| | process-sockets-141 (seconds) | 20.8735 | (I) -4.27% |
| | process-sockets-172 (seconds) | 26.205333333333332 | 0.30% |
| | process-sockets-203 (seconds) | 31.298000000000002 | -1.71% |
| | process-sockets-234 (seconds) | 36.104000000000006 | -1.94% |
| | process-sockets-256 (seconds) | 39.44016666666667 | -0.71% |
| | thread-pipes-1 (seconds) | 0.17550000000000002 | 0.66% |
| | thread-pipes-4 (seconds) | 0.44716666666666666 | 1.66% |
| | thread-pipes-7 (seconds) | 0.7345 | -0.17% |
| | thread-pipes-12 (seconds) | 1.405833333333333 | (I) -4.12% |
| | thread-pipes-21 (seconds) | 2.0113333333333334 | (I) -2.13% |
| | thread-pipes-30 (seconds) | 2.6648333333333336 | (I) -3.78% |
| | thread-pipes-48 (seconds) | 3.6341666666666668 | (I) -5.77% |
| | thread-pipes-79 (seconds) | 4.4085 | (I) -5.31% |
| | thread-pipes-110 (seconds) | 5.374666666666666 | (I) -6.12% |
| | thread-pipes-141 (seconds) | 6.385666666666666 | (I) -4.00% |
| | thread-pipes-172 (seconds) | 7.403000000000001 | (I) -3.01% |
| | thread-pipes-203 (seconds) | 8.570333333333332 | (I) -2.62% |
| | thread-pipes-234 (seconds) | 9.719166666666666 | (I) -2.00% |
| | thread-pipes-256 (seconds) | 10.552833333333334 | (I) -2.30% |
| | thread-sockets-1 (seconds) | 0.3065 | (R) 2.39% |
+------------------------------------+----------------------------------------------------------+-----------------------+--------------------------+

+------------------------------------+----------------------------------------------------------+-----------------------+--------------------------+
| mmtests/sysbench-mutex | sysbenchmutex-1 (usec) | 194.38333333333333 | -0.02% |
| | sysbenchmutex-4 (usec) | 200.875 | -0.02% |
| | sysbenchmutex-7 (usec) | 201.23000000000002 | 0.00% |
| | sysbenchmutex-12 (usec) | 201.77666666666664 | 0.12% |
| | sysbenchmutex-21 (usec) | 203.03 | -0.40% |
| | sysbenchmutex-30 (usec) | 203.285 | 0.08% |
| | sysbenchmutex-48 (usec) | 231.30000000000004 | 2.59% |
| | sysbenchmutex-79 (usec) | 362.075 | -0.80% |
| | sysbenchmutex-110 (usec) | 516.8233333333334 | -3.87% |
| | sysbenchmutex-128 (usec) | 593.3533333333334 | (I) -4.46% |
+------------------------------------+----------------------------------------------------------+-----------------------+--------------------------+

mm/khugepaged.c | 9 ++-------
1 file changed, 2 insertions(+), 7 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 4ec324a4c1fe..a0f1df2a7ae6 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -676,9 +676,7 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
writable = true;
}

- if (unlikely(!writable)) {
- result = SCAN_PAGE_RO;
- } else if (unlikely(cc->is_khugepaged && !referenced)) {
+ if (unlikely(cc->is_khugepaged && !referenced)) {
result = SCAN_LACK_REFERENCED_PAGE;
} else {
result = SCAN_SUCCEED;
@@ -1421,9 +1419,7 @@ static int hpage_collapse_scan_pmd(struct mm_struct *mm,
mmu_notifier_test_young(vma->vm_mm, _address)))
referenced++;
}
- if (!writable) {
- result = SCAN_PAGE_RO;
- } else if (cc->is_khugepaged &&
+ if (cc->is_khugepaged &&
(!referenced ||
(unmapped && referenced < HPAGE_PMD_NR / 2))) {
result = SCAN_LACK_REFERENCED_PAGE;
@@ -2830,7 +2826,6 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
case SCAN_PMD_NULL:
case SCAN_PTE_NON_PRESENT:
case SCAN_PTE_UFFD_WP:
- case SCAN_PAGE_RO:
case SCAN_LACK_REFERENCED_PAGE:
case SCAN_PAGE_NULL:
case SCAN_PAGE_COUNT:
--
2.30.2