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Fri, 02 Oct 2026 17:21:48 -0700 (PDT) Date: Sat, 3 Oct 2026 00:21:19 +0000 In-Reply-To: <20261003002123.505555-1-jthoughton@google.com> Mime-Version: 1.0 References: <20261003002123.505555-1-jthoughton@google.com> X-Mailer: git-send-email 2.56.0.rc1.315.gc6ed9934b7-goog Message-ID: <20261003002123.505555-17-jthoughton@google.com> Subject: [PATCH v2 16/20 DO-NOT-MERGE] hugetlb_vmemmap: Use try_populate_vmemmap_pmd for replacing in-use PMDs From: James Houghton To: Will Deacon , Catalin Marinas , Muchun Song , Oscar Salvador , Andrew Morton Cc: Nikos Nikoleris , Linu Cherian , Mark Rutland , David Hildenbrand , Ryan Roberts , Nanyong Sun , Yu Zhao , Frank van der Linden , David Rientjes , James Houghton , linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org Content-Type: text/plain; charset="UTF-8" X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.9.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20261002_172150_343855_88C9508A X-CRM114-Status: GOOD ( 22.14 ) 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: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org This routine is to be used for updating in-use, leaf-level PMDs in the vmemmap without introducing a window where vmemmap accesses might fault. Because try_populate_vmemmap_pmd() can fail, the split_page() call needs to be moved to after the PMD update. Because both split_page() and the PMD update are done under the page table lock, this rearrangement is safe. Only leaf-level PMD to non-leaf PMD (mapping the same physical pages) is the only transition that needs to be supported. Collapsing a non-leaf PMD to a leaf-level PMD is not used by HVO, so try_populate_vmemmap_pmd() need not support it. This patch allows arm64's implementation to replaced with one that does not require BBML3. Signed-off-by: James Houghton --- arch/arm64/include/asm/pgalloc.h | 8 ++++++++ arch/loongarch/include/asm/pgalloc.h | 2 ++ arch/riscv/include/asm/pgalloc.h | 2 ++ arch/x86/include/asm/pgalloc.h | 2 ++ include/linux/pgalloc.h | 21 +++++++++++++++++++++ mm/hugetlb_vmemmap.c | 25 ++++++++++++++----------- 6 files changed, 49 insertions(+), 11 deletions(-) diff --git a/arch/arm64/include/asm/pgalloc.h b/arch/arm64/include/asm/pgalloc.h index 1b4509d3382c..0263329fb7b3 100644 --- a/arch/arm64/include/asm/pgalloc.h +++ b/arch/arm64/include/asm/pgalloc.h @@ -121,4 +121,12 @@ pmd_populate(struct mm_struct *mm, pmd_t *pmdp, pgtable_t ptep) PMD_TYPE_TABLE | PMD_TABLE_AF | PMD_TABLE_PXN); } +static inline int try_populate_vmemmap_pmd(unsigned long addr, pmd_t *pmdp, + pte_t *pgtable) +{ + /* BBLM3 is required. Its presence has been checked. */ + pmd_populate_kernel(&init_mm, pmdp, pgtable); + return 0; +} + #endif diff --git a/arch/loongarch/include/asm/pgalloc.h b/arch/loongarch/include/asm/pgalloc.h index 248f62d0b590..0553addb0afa 100644 --- a/arch/loongarch/include/asm/pgalloc.h +++ b/arch/loongarch/include/asm/pgalloc.h @@ -24,6 +24,8 @@ static inline void pmd_populate(struct mm_struct *mm, pmd_t *pmd, pgtable_t pte) set_pmd(pmd, __pmd((unsigned long)page_address(pte))); } +#define ARCH_WANTS_GENERIC_POPULATE_VMEMMAP_PMD + #ifndef __PAGETABLE_PMD_FOLDED static inline void pud_populate(struct mm_struct *mm, pud_t *pud, pmd_t *pmd) diff --git a/arch/riscv/include/asm/pgalloc.h b/arch/riscv/include/asm/pgalloc.h index 770ce18a7328..b9b4b25fc6f0 100644 --- a/arch/riscv/include/asm/pgalloc.h +++ b/arch/riscv/include/asm/pgalloc.h @@ -31,6 +31,8 @@ static inline void pmd_populate(struct mm_struct *mm, set_pmd(pmd, __pmd((pfn << _PAGE_PFN_SHIFT) | _PAGE_TABLE)); } +#define ARCH_WANTS_GENERIC_POPULATE_VMEMMAP_PMD + #ifndef __PAGETABLE_PMD_FOLDED static inline void pud_populate(struct mm_struct *mm, pud_t *pud, pmd_t *pmd) { diff --git a/arch/x86/include/asm/pgalloc.h b/arch/x86/include/asm/pgalloc.h index c88691b15f3c..f8d24b97cbf5 100644 --- a/arch/x86/include/asm/pgalloc.h +++ b/arch/x86/include/asm/pgalloc.h @@ -82,6 +82,8 @@ static inline void pmd_populate(struct mm_struct *mm, pmd_t *pmd, set_pmd(pmd, __pmd(((pteval_t)pfn << PAGE_SHIFT) | _PAGE_TABLE)); } +#define ARCH_WANTS_GENERIC_POPULATE_VMEMMAP_PMD + #if CONFIG_PGTABLE_LEVELS > 2 extern void ___pmd_free_tlb(struct mmu_gather *tlb, pmd_t *pmd); diff --git a/include/linux/pgalloc.h b/include/linux/pgalloc.h index 9174fa59bbc5..1dbecb748028 100644 --- a/include/linux/pgalloc.h +++ b/include/linux/pgalloc.h @@ -26,4 +26,25 @@ arch_sync_kernel_mappings(addr, addr); \ } while (0) +#ifdef ARCH_WANTS_GENERIC_POPULATE_VMEMMAP_PMD +/* + * try_populate_vmemmap_pmd - Populate a PMD that is in use by the vmemmap. + * @addr: Base address of the remapped PMD. + * @pmdp: Page table pointer to be overwritten. + * @pgtable: Pointer to the page table that the new PMD will point to. + * + * This function is only to be used to update PMDs that map the vmemmap to + * point to a page of already-populated PTEs that map the same pages. + * + * Implementations of this function must ensure that, while the update is taking + * place, CPUs will not fault on the remapped virtual address range. + */ +static inline int try_populate_vmemmap_pmd(unsigned long addr, pmd_t *pmdp, + pte_t *pgtable) +{ + pmd_populate_kernel(&init_mm, pmdp, pgtable); + return 0; +} +#endif + #endif /* _LINUX_PGALLOC_H */ diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c index 1eca03a3def3..9e0f52474bdb 100644 --- a/mm/hugetlb_vmemmap.c +++ b/mm/hugetlb_vmemmap.c @@ -88,6 +88,7 @@ static int vmemmap_split_pmd(pmd_t *pmd, struct page *head, unsigned long start, struct vmemmap_remap_walk *walk) { pmd_t __pmd; + int ret; int i; unsigned long addr = start; pte_t *pgtable; @@ -107,8 +108,15 @@ static int vmemmap_split_pmd(pmd_t *pmd, struct page *head, unsigned long start, set_pte_at(&init_mm, addr, pte, entry); } + ret = 0; spin_lock(&init_mm.page_table_lock); if (likely(pmd_leaf(*pmd))) { + /* Make pte visible before pmd. See comment in pmd_install(). */ + smp_wmb(); + ret = try_populate_vmemmap_pmd(start, pmd, pgtable); + if (ret) + goto free; + /* * Higher order allocations from buddy allocator must be able to * be treated as independent small pages (as they can be freed @@ -117,21 +125,16 @@ static int vmemmap_split_pmd(pmd_t *pmd, struct page *head, unsigned long start, if (!PageReserved(head)) split_page(head, get_order(PMD_SIZE)); - /* Make pte visible before pmd. See comment in pmd_install(). */ - smp_wmb(); - /* - * On arm64, this requires BBML3. Its support has already been - * checked. - */ - pmd_populate_kernel(&init_mm, pmd, pgtable); if (!(walk->flags & VMEMMAP_SPLIT_NO_TLB_FLUSH)) flush_tlb_kernel_range(start, start + PMD_SIZE); - } else { - pte_free_kernel(&init_mm, pgtable); + goto out; } - spin_unlock(&init_mm.page_table_lock); - return 0; +free: + pte_free_kernel(&init_mm, pgtable); +out: + spin_unlock(&init_mm.page_table_lock); + return ret; } static int vmemmap_pmd_entry(pmd_t *pmd, unsigned long addr, -- 2.56.0.rc1.315.gc6ed9934b7-goog