6.6-stable review patch. If anyone has any objections, please let me know.
------------------
From: Huang Ying ying.huang@linux.alibaba.com
[ 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 ying.huang@linux.alibaba.com Cc: Will Deacon will@kernel.org Cc: Anshuman Khandual anshuman.khandual@arm.com Cc: Ryan Roberts ryan.roberts@arm.com Cc: Gavin Shan gshan@redhat.com Cc: Ard Biesheuvel ardb@kernel.org Cc: Matthew Wilcox (Oracle) willy@infradead.org Cc: Yicong Yang yangyicong@hisilicon.com Cc: linux-arm-kernel@lists.infradead.org Cc: linux-kernel@vger.kernel.org Reviewed-by: Catalin Marinas catalin.marinas@arm.com Signed-off-by: Catalin Marinas catalin.marinas@arm.com Signed-off-by: Sasha Levin sashal@kernel.org --- arch/arm64/include/asm/pgtable.h | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h index 0212129b13d07..92e43b3a10df9 100644 --- a/arch/arm64/include/asm/pgtable.h +++ b/arch/arm64/include/asm/pgtable.h @@ -184,7 +184,8 @@ static inline pmd_t set_pmd_bit(pmd_t pmd, pgprot_t prot) static inline pte_t pte_mkwrite_novma(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; }