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From: Muchun Song In-Reply-To: Date: Fri, 3 Feb 2023 10:40:18 +0800 Cc: Will Deacon , Robin Murphy , Anshuman Khandual , linux-arm-kernel@lists.infradead.org, Mark Rutland , Andrew Morton , linux-kernel@vger.kernel.org, Mark Brown Message-Id: References: <20230109052816.405335-1-anshuman.khandual@arm.com> <20230126133321.GB29148@willie-the-truck> <20230131154950.GB2646@willie-the-truck> To: Catalin Marinas X-Migadu-Flow: FLOW_OUT X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20230202_184107_259731_1E0E6071 X-CRM114-Status: GOOD ( 21.69 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org > On Feb 2, 2023, at 18:45, Catalin Marinas wrote: > > On Thu, Feb 02, 2023 at 05:51:39PM +0800, Muchun Song wrote: >>> On Feb 1, 2023, at 20:20, Catalin Marinas wrote: >>>> Bah, sorry! Catalin reckons it may have been him talking about the vmemmap. >>> >>> Indeed. The discussion with Anshuman started from this thread: >>> >>> https://lore.kernel.org/all/20221025014215.3466904-1-mawupeng1@huawei.com/ >>> >>> We already trip over the existing checks even without Anshuman's patch, >>> though only by chance. We are not setting the software PTE_DIRTY on the >>> new pte (we don't bother with this bit for kernel mappings). >>> >>> Given that the vmemmap ptes are still live when such change happens and >>> no-one came with a solution to the break-before-make problem, I propose >>> we revert the arm64 part of commit 47010c040dec ("mm: hugetlb_vmemmap: >>> cleanup CONFIG_HUGETLB_PAGE_FREE_VMEMMAP*"). We just need this hunk: >>> >>> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig >>> index 27b2592698b0..5263454a5794 100644 >>> --- a/arch/arm64/Kconfig >>> +++ b/arch/arm64/Kconfig >>> @@ -100,7 +100,6 @@ config ARM64 >>> select ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT >>> select ARCH_WANT_FRAME_POINTERS >>> select ARCH_WANT_HUGE_PMD_SHARE if ARM64_4K_PAGES || (ARM64_16K_PAGES && !ARM64_VA_BITS_36) >>> - select ARCH_WANT_HUGETLB_PAGE_OPTIMIZE_VMEMMAP >> >> Maybe it is a little overkill for HVO as it can significantly minimize the >> overhead of vmemmap on ARM64 servers for some workloads (like qemu, DPDK). >> So I don't think disabling it is a good approach. Indeed, HVO broke BBM, >> but the waring does not affect anything since the tail vmemmap pages are >> supposed to be read-only. So, I suggest skipping warnings if it is the >> vmemmap address in set_pte_at(). What do you think of? > > IIUC, vmemmap_remap_pte() not only makes the pte read-only but also > changes the output address. Architecturally, this needs a BBM sequence. > We can avoid going through an invalid pte if we first make the pte > read-only, TLBI but keeping the same pfn, followed by a change of the > pfn while keeping the pte readonly. This also assumes that the content > of the page pointed at by the pte is the same at both old and new pfn. Right. I think using BBM is to avoid possibly creating multiple TLB entries for the same address for a extremely short period. But accessing either the old page or the new page is fine in this case. Is it acceptable for this special case without using BBM? Thanks, Muchun. _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel