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

Zi Yan
2025-09-03T15:46:42Z

On 3 Sep 2025, at 1:46, Dev Jain wrote:

> 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(-)
>

Acked-by: Zi Yan <ziy@nvidia.com>

Best Regards,
Yan, Zi