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From: Muchun Song In-Reply-To: <5fac578c-ff97-4721-96f3-8ba495f0556a@iscas.ac.cn> Date: Mon, 6 Jul 2026 13:38:06 +0800 Cc: linuxppc-dev@lists.ozlabs.org, Paul Walmsley , Palmer Dabbelt , Alexandre Ghiti , Andrew Morton , David Hildenbrand , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Madhavan Srinivasan , Michael Ellerman , Nicholas Piggin , "Christophe Leroy (CS GROUP)" Message-Id: <214FC970-465A-4E45-98C0-17978D63DE89@linux.dev> References: <20260630-mark-after-vmemmap-populate-v4-1-febbc15da028@iscas.ac.cn> <7B0CB4ED-C190-4F49-97EE-E1F14F08DED2@linux.dev> <5fac578c-ff97-4721-96f3-8ba495f0556a@iscas.ac.cn> To: Vivian Wang X-Migadu-Flow: FLOW_OUT X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.9.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20260705_223853_403874_4F275961 X-CRM114-Status: GOOD ( 31.38 ) X-BeenThere: linux-riscv@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-riscv" Errors-To: linux-riscv-bounces+linux-riscv=archiver.kernel.org@lists.infradead.org > On Jul 6, 2026, at 13:04, Vivian Wang wrote: > > [ powerpc maintainers: Thoughts on replacing the ptesync in > vmemmap_set_pmd() with a flush_cache_vmap() post-vmemmap-hotplug in the > generic code? ] > > On 7/6/26 11:00, Muchun Song wrote: >>> On Jun 30, 2026, at 15:51, Vivian Wang wrote: >>> >>> section_activate() does not flush TLB after populating new vmemmap >>> pages. On most architectures, this is okay. However it is a problem on >>> RISC-V since there the TLB caching non-present entries is permitted, >>> which causes spurious faults on some hardwares. >>> >>> This seems to be most easily reproduced with DEBUG_VM=y and >>> PAGE_POISONING=y, which causes these newly mapped struct pages to be >>> poisoned i.e. written to immediately after mapping. >>> >>> Add a hook vmemmap_populate_finalize() in __populate_section_memmap() >>> after population, to allow architectures to handle such situations as >>> needed. Then implement it on RISC-V to arrange for the existing >>> exception handler code to deal with these faults if they happen. >>> >>> Signed-off-by: Vivian Wang >>> --- >>> Changes in v4: >>> - Rebase on v7.2-rc1, drop dependencies >>> - (No code changes otherwise) >>> - (A concurrency fix for mark_new_valid_map was sent independently) >>> https://lore.kernel.org/linux-riscv/20260629-riscv-mm-new-valid-map-ordering-v1-1-60d8c10c6292@iscas.ac.cn/ >>> - Link to v3: https://patch.msgid.link/20260605-mark-after-vmemmap-populate-v3-1-a06001ac9264@iscas.ac.cn >>> >>> Changes in v3: >>> - Merged back into one patch (Mike) >>> - (No code changes otherwise.) >>> - Link to v2: https://patch.msgid.link/20260604-mark-after-vmemmap-populate-v2-0-ab6a7d03b434@iscas.ac.cn >>> >>> Changes in v2: >>> - Split patch in two, hook point and riscv hook >>> - Explain hook necessity in patch 1 message (Mike) >>> - Make hook #define based (Mike) >>> - Call finalize hook only on populate success >>> - Link to v1: https://patch.msgid.link/20260525-mark-after-vmemmap-populate-v1-1-e698d859ba16@iscas.ac.cn >>> --- >>> arch/riscv/include/asm/pgtable.h | 4 ++++ >>> arch/riscv/mm/init.c | 6 ++++++ >>> mm/sparse-vmemmap.c | 8 ++++++++ >>> 3 files changed, 18 insertions(+) >>> >>> diff --git a/arch/riscv/include/asm/pgtable.h b/arch/riscv/include/asm/pgtable.h >>> index 5d5756bda82e..6b000c990ba7 100644 >>> --- a/arch/riscv/include/asm/pgtable.h >>> +++ b/arch/riscv/include/asm/pgtable.h >>> @@ -1253,6 +1253,10 @@ static inline pte_t pte_swp_clear_exclusive(pte_t pte) >>> #define TASK_SIZE FIXADDR_START >>> #endif >>> >>> +/* Needed on SPARSEMEM_VMEMMAP */ >>> +#define vmemmap_populate_finalize vmemmap_populate_finalize >>> +void __meminit vmemmap_populate_finalize(void); >>> + >>> #else /* CONFIG_MMU */ >>> >>> #define PAGE_SHARED __pgprot(0) >>> diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c >>> index 5b1b3c88b4d1..800cb5c007d1 100644 >>> --- a/arch/riscv/mm/init.c >>> +++ b/arch/riscv/mm/init.c >>> @@ -1372,6 +1372,12 @@ int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node, >>> */ >>> return vmemmap_populate_hugepages(start, end, node, altmap); >>> } >>> + >>> +void __meminit vmemmap_populate_finalize(void) >>> +{ >>> + /* Avoid faults on cached non-present TLB entries. */ >>> + mark_new_valid_map(); >>> +} >>> #endif >>> >>> #if defined(CONFIG_MMU) && defined(CONFIG_64BIT) >>> diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c >>> index 99e2be39671b..290cafcfd723 100644 >>> --- a/mm/sparse-vmemmap.c >>> +++ b/mm/sparse-vmemmap.c >>> @@ -544,6 +544,12 @@ static int __meminit vmemmap_populate_compound_pages(unsigned long start_pfn, >>> >>> #endif >>> >>> +#ifndef vmemmap_populate_finalize >>> +static void __meminit vmemmap_populate_finalize(void) >>> +{ >>> +} >>> +#endif >>> + >>> struct page * __meminit __populate_section_memmap(unsigned long pfn, >>> unsigned long nr_pages, int nid, struct vmem_altmap *altmap, >>> struct dev_pagemap *pgmap) >>> @@ -564,6 +570,8 @@ struct page * __meminit __populate_section_memmap(unsigned long pfn, >>> if (r < 0) >>> return NULL; >>> >>> + vmemmap_populate_finalize(); >> Does the hook of flush_cache_vmap() work for you? From the document of >> cachetlb.rst, it said: >> >> 6) ``void flush_cache_vmap(unsigned long start, unsigned long end)`` >> ``void flush_cache_vunmap(unsigned long start, unsigned long end)`` >> >> Here in these two interfaces we are flushing a specific range >> of (kernel) virtual addresses from the cache. After running, >> there will be no entries in the cache for the kernel address >> space for virtual addresses in the range 'start' to 'end-1'. >> >> It seems that flush_cache_vmap() is supposed to be called after a new kernel >> mapping has been installed, but before this address range is accessed. > > Good point. I had initially avoided flush_cache_vmap() because, to be > honest, it looked scary. But the combination of "have vmemmap" and "have > flush_cache_vmap()" is much less scary. > > I'd need to drop the address range check in RISC-V's flush_cache_vmap(), > but might as well. No. Please a new check for vmemmap address range. > >> Therefore, invoking flush_cache_vmap() here aligns perfectly with its interface >> definition. But how would this change impact other architectures? Currently, >> among architectures where CONFIG_SPARSEMEM=y, only RISC-V and PowerPC provide >> a concrete implementation for flush_cache_vmap(). >> >> For PowerPC, the implementation is as follows: >> >> static inline void flush_cache_vmap(unsigned long start, unsigned long end) >> { >> asm volatile("ptesync" ::: "memory"); >> } >> >> Here, flush_cache_vmap() is also intended to supply the ptesync instruction >> that the kernel address mapping lacks. However, flush_cache_vmap() is not >> called in the __populate_section_memmap() path. So, how does PowerPC ensure >> ptesync is executed there? The answer lies in the architecture-specific >> vmemmap_set_pmd(), which injects the necessary ptesync. >> >> If we choose to reuse flush_cache_vmap() in the generic path, it yields the >> following benefits: >> >> - Avoids architectural churn: We do not need to introduce a new hook >> like vmemmap_populate_finalize(). >> - Cleans up arch code: PowerPC can deprecate its custom vmemmap_set_pmd() >> and migrate to the generic implementation. >> >> Let me know if this rationale makes sense, or if there are any hidden edge >> cases I might have overlooked regarding other architectures. >> >> Thanks, >> Muchun > > At least on the face of it, it makes sense. The main thing is I don't > know much about powerpc. So I'm also hoping powerpc maintainers could > chime in. You do not need to know about powerpc since the change of powerpc is not a part of this patch. It should be a separate code clean up for powerpc. > > I also don't know how this would affect new archs, but if they need > flush_cache_vmap() they *probably* also want this treatment for vmemmap > hotplug? I guess we can deal with that when it happens. > > Thanks, > Vivian "dramforever" Wang _______________________________________________ linux-riscv mailing list linux-riscv@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-riscv