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Mon, 20 Oct 2025 19:00:30 +0800 From: "Huang, Ying" To: Catalin Marinas Cc: Will Deacon , Anshuman Khandual , Ryan Roberts , Gavin Shan , Ard Biesheuvel , "Matthew Wilcox (Oracle)" , Yicong Yang , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH] arm64, mm: avoid always making PTE dirty in pte_mkwrite() In-Reply-To: (Catalin Marinas's message of "Fri, 17 Oct 2025 19:06:16 +0100") References: <20251015023712.46598-1-ying.huang@linux.alibaba.com> Date: Mon, 20 Oct 2025 19:00:20 +0800 Message-ID: <87bjm1g7nf.fsf@DESKTOP-5N7EMDA> User-Agent: Gnus/5.13 (Gnus v5.13) MIME-Version: 1.0 Content-Type: text/plain; charset=ascii X-Rspamd-Queue-Id: 1DD3C16001D X-Rspamd-Server: rspam11 X-Rspam-User: X-Stat-Signature: irfwawjx86uomjxe31dsc3kg6zqugosy X-HE-Tag: 1760958035-821780 X-HE-Meta: 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 JqGl2v2X 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Hi, Catalin, Catalin Marinas writes: > On Wed, Oct 15, 2025 at 10:37:12AM +0800, Huang Ying wrote: >> 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: Catalin Marinas >> 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 >> --- >> 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 aa89c2e67ebc..0944e296dd4a 100644 >> --- a/arch/arm64/include/asm/pgtable.h >> +++ b/arch/arm64/include/asm/pgtable.h >> @@ -293,7 +293,8 @@ 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; >> } > > This seems to be the right thing. I recall years ago I grep'ed > (obviously not hard enough) and most pte_mkwrite() places had a > pte_mkdirty(). But I missed do_swap_page() and possibly others. The do_swap_page() change was introduced in June 2024, quite recently. > For this patch: > > Reviewed-by: Catalin Marinas Thanks! > I wonder whether we should also add (as a separate patch): > > diff --git a/mm/debug_vm_pgtable.c b/mm/debug_vm_pgtable.c > index 830107b6dd08..df1c552ef11c 100644 > --- a/mm/debug_vm_pgtable.c > +++ b/mm/debug_vm_pgtable.c > @@ -101,6 +101,7 @@ static void __init pte_basic_tests(struct pgtable_debug_args *args, int idx) > WARN_ON(pte_dirty(pte_mkclean(pte_mkdirty(pte)))); > WARN_ON(pte_write(pte_wrprotect(pte_mkwrite(pte, args->vma)))); > WARN_ON(pte_dirty(pte_wrprotect(pte_mkclean(pte)))); > + WARN_ON(pte_dirty(pte_mkwrite_novma(pte_mkclean(pte)))); > WARN_ON(!pte_dirty(pte_wrprotect(pte_mkdirty(pte)))); > } > > For completeness, also (and maybe other combinations): > > WARN_ON(!pte_write(pte_mkdirty(pte_mkwrite_novma(pte)))); Sure. Will add another patch for this. > I cc'ed linux-mm in case we missed anything. If nothing raised, I'll > queue it next week. --- Best Regards, Huang, Ying