mm: enable khugepaged anonymous collapse on non-writable regions

mm: enable khugepaged anonymous collapse on non-writable regions

Patch series "Expand scope of khugepaged anonymous collapse", v2.

Currently khugepaged does not collapse an anonymous region which does not
have a single writable pte.  This is wasteful since a region mapped with
non-writable ptes, for example, non-writable VMAs mapped by the
application, won't benefit from THP collapse.

An additional consequence of this constraint is that 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 overridden by the kernel.

Therefore, remove this constraint.

On an arm64 bare metal machine, comparing with vanilla 6.17-rc2, an
average of 5% improvement is seen on some mmtests benchmarks, particularly
hackbench, with a maximum improvement of 12%.  In the following table, (I)
denotes statistically significant improvement, (R) denotes statistically
significant regression.

+-------------------------+--------------------------------+---------------+
| mmtests/hackbench       | process-pipes-1 (seconds)      |        -0.06% |
|                         | process-pipes-4 (seconds)      |        -0.27% |
|                         | process-pipes-7 (seconds)      |   (I) -12.13% |
|                         | process-pipes-12 (seconds)     |    (I) -5.32% |
|                         | process-pipes-21 (seconds)     |    (I) -2.87% |
|                         | process-pipes-30 (seconds)     |    (I) -3.39% |
|                         | process-pipes-48 (seconds)     |    (I) -5.65% |
|                         | process-pipes-79 (seconds)     |    (I) -6.74% |
|                         | process-pipes-110 (seconds)    |    (I) -6.26% |
|                         | process-pipes-141 (seconds)    |    (I) -4.99% |
|                         | process-pipes-172 (seconds)    |    (I) -4.45% |
|                         | process-pipes-203 (seconds)    |    (I) -3.65% |
|                         | process-pipes-234 (seconds)    |    (I) -3.45% |
|                         | process-pipes-256 (seconds)    |    (I) -3.47% |
|                         | process-sockets-1 (seconds)    |         2.13% |
|                         | process-sockets-4 (seconds)    |         1.02% |
|                         | process-sockets-7 (seconds)    |        -0.26% |
|                         | process-sockets-12 (seconds)   |        -1.24% |
|                         | process-sockets-21 (seconds)   |         0.01% |
|                         | process-sockets-30 (seconds)   |        -0.15% |
|                         | process-sockets-48 (seconds)   |         0.15% |
|                         | process-sockets-79 (seconds)   |         1.45% |
|                         | process-sockets-110 (seconds)  |        -1.64% |
|                         | process-sockets-141 (seconds)  |    (I) -4.27% |
|                         | process-sockets-172 (seconds)  |         0.30% |
|                         | process-sockets-203 (seconds)  |        -1.71% |
|                         | process-sockets-234 (seconds)  |        -1.94% |
|                         | process-sockets-256 (seconds)  |        -0.71% |
|                         | thread-pipes-1 (seconds)       |         0.66% |
|                         | thread-pipes-4 (seconds)       |         1.66% |
|                         | thread-pipes-7 (seconds)       |        -0.17% |
|                         | thread-pipes-12 (seconds)      |    (I) -4.12% |
|                         | thread-pipes-21 (seconds)      |    (I) -2.13% |
|                         | thread-pipes-30 (seconds)      |    (I) -3.78% |
|                         | thread-pipes-48 (seconds)      |    (I) -5.77% |
|                         | thread-pipes-79 (seconds)      |    (I) -5.31% |
|                         | thread-pipes-110 (seconds)     |    (I) -6.12% |
|                         | thread-pipes-141 (seconds)     |    (I) -4.00% |
|                         | thread-pipes-172 (seconds)     |    (I) -3.01% |
|                         | thread-pipes-203 (seconds)     |    (I) -2.62% |
|                         | thread-pipes-234 (seconds)     |    (I) -2.00% |
|                         | thread-pipes-256 (seconds)     |    (I) -2.30% |
|                         | thread-sockets-1 (seconds)     |     (R) 2.39% |
+-------------------------+--------------------------------+---------------+

+-------------------------+------------------------------------------------+
| mmtests/sysbench-mutex  | sysbenchmutex-1 (usec)         |        -0.02% |
|                         | sysbenchmutex-4 (usec)         |        -0.02% |
|                         | sysbenchmutex-7 (usec)         |         0.00% |
|                         | sysbenchmutex-12 (usec)        |         0.12% |
|                         | sysbenchmutex-21 (usec)        |        -0.40% |
|                         | sysbenchmutex-30 (usec)        |         0.08% |
|                         | sysbenchmutex-48 (usec)        |         2.59% |
|                         | sysbenchmutex-79 (usec)        |        -0.80% |
|                         | sysbenchmutex-110 (usec)       |        -3.87% |
|                         | sysbenchmutex-128 (usec)       |    (I) -4.46% |
+-------------------------+--------------------------------+---------------+

This patch (of 2):

Currently khugepaged does not collapse an anonymous region which does not
have a single writable pte.  This is wasteful since a region mapped with
non-writable ptes, for example, non-writable VMAs mapped by the
application, won't benefit from THP collapse.

An additional consequence of this constraint is that 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 overridden by the kernel.

Therefore, remove this restriction by not honouring SCAN_PAGE_RO.

Link: https://lkml.kernel.org/r/20250908075028.38431-1-dev.jain@arm.com
Link: https://lkml.kernel.org/r/20250908075028.38431-2-dev.jain@arm.com
Signed-off-by: Dev Jain <dev.jain@arm.com>
Acked-by: David Hildenbrand <david@redhat.com>
Acked-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Wei Yang <richard.weiyang@gmail.com>
Reviewed-by: Kiryl Shutsemau <kas@kernel.org>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Reviewed-by: Zach O'Keefe <zokeefe@google.com>
Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Mariano Pache <npache@redhat.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>