From: Huang Ying Date: Wed, 15 Oct 2025 02:37:12 +0000 (+0800) Subject: arm64, mm: avoid always making PTE dirty in pte_mkwrite() X-Git-Tag: v5.15.196~57 X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=8a2375b0e9b89c5fc7aa41771e906415c25ac095;p=thirdparty%2Fkernel%2Fstable.git arm64, mm: avoid always making PTE dirty in pte_mkwrite() [ Upstream commit 143937ca51cc6ae2fccc61a1cb916abb24cd34f5 ] Current pte_mkwrite_novma() makes PTE dirty unconditionally. This may mark some pages that are never written dirty wrongly. For example, do_swap_page() may map the exclusive pages with writable and clean PTEs if the VMA is writable and the page fault is for read access. However, current pte_mkwrite_novma() implementation always dirties the PTE. This may cause unnecessary disk writing if the pages are never written before being reclaimed. So, change pte_mkwrite_novma() to clear the PTE_RDONLY bit only if the PTE_DIRTY bit is set to make it possible to make the PTE writable and clean. The current behavior was introduced in commit 73e86cb03cf2 ("arm64: Move PTE_RDONLY bit handling out of set_pte_at()"). Before that, pte_mkwrite() only sets the PTE_WRITE bit, while set_pte_at() only clears the PTE_RDONLY bit if both the PTE_WRITE and the PTE_DIRTY bits are set. To test the performance impact of the patch, on an arm64 server machine, run 16 redis-server processes on socket 1 and 16 memtier_benchmark processes on socket 0 with mostly get transactions (that is, redis-server will mostly read memory only). The memory footprint of redis-server is larger than the available memory, so swap out/in will be triggered. Test results show that the patch can avoid most swapping out because the pages are mostly clean. And the benchmark throughput improves ~23.9% in the test. Fixes: 73e86cb03cf2 ("arm64: Move PTE_RDONLY bit handling out of set_pte_at()") Signed-off-by: Huang Ying Cc: Will Deacon Cc: Anshuman Khandual Cc: Ryan Roberts Cc: Gavin Shan Cc: Ard Biesheuvel Cc: Matthew Wilcox (Oracle) Cc: Yicong Yang Cc: linux-arm-kernel@lists.infradead.org Cc: linux-kernel@vger.kernel.org Reviewed-by: Catalin Marinas Signed-off-by: Catalin Marinas Signed-off-by: Sasha Levin --- diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h index a0bfa9cd76dab..a1902dcf7a7e3 100644 --- a/arch/arm64/include/asm/pgtable.h +++ b/arch/arm64/include/asm/pgtable.h @@ -175,7 +175,8 @@ static inline pmd_t set_pmd_bit(pmd_t pmd, pgprot_t prot) static inline pte_t pte_mkwrite(pte_t pte) { pte = set_pte_bit(pte, __pgprot(PTE_WRITE)); - pte = clear_pte_bit(pte, __pgprot(PTE_RDONLY)); + if (pte_sw_dirty(pte)) + pte = clear_pte_bit(pte, __pgprot(PTE_RDONLY)); return pte; }