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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>