On Mon, Mar 11, 2019 at 01:58:27PM +0530, Anshuman Khandual wrote:

On 03/10/2019 06:49 AM, Yu Zhao wrote:

quoted

Switch from per mm_struct to per pmd page table lock by enabling
ARCH_ENABLE_SPLIT_PMD_PTLOCK. This provides better granularity for
large system.

I'm not sure if there is contention on mm->page_table_lock. Given
the option comes at no cost (apart from initializing more spin
locks), why not enable it now.

We only do so when pmd is not folded, so we don't mistakenly call
pgtable_pmd_page_ctor() on pud or p4d in pgd_pgtable_alloc(). (We
check shift against PMD_SHIFT, which is same as PUD_SHIFT when pmd
is folded).

Signed-off-by: Yu Zhao <redacted>
---
 arch/arm64/Kconfig               |  3 +++
 arch/arm64/include/asm/pgalloc.h | 12 +++++++++++-
 arch/arm64/include/asm/tlb.h     |  5 ++++-
 3 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index cfbf307d6dc4..a3b1b789f766 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig

@@ -872,6 +872,9 @@ config ARCH_WANT_HUGE_PMD_SHARE
 config ARCH_HAS_CACHE_LINE_SIZE
 	def_bool y
 
+config ARCH_ENABLE_SPLIT_PMD_PTLOCK
+	def_bool y if PGTABLE_LEVELS > 2
+
 config SECCOMP
 	bool "Enable seccomp to safely compute untrusted bytecode"
 	---help---

diff --git a/arch/arm64/include/asm/pgalloc.h b/arch/arm64/include/asm/pgalloc.h
index 52fa47c73bf0..dabba4b2c61f 100644
--- a/arch/arm64/include/asm/pgalloc.h
+++ b/arch/arm64/include/asm/pgalloc.h

@@ -33,12 +33,22 @@
 
 static inline pmd_t *pmd_alloc_one(struct mm_struct *mm, unsigned long addr)
 {
-	return (pmd_t *)__get_free_page(PGALLOC_GFP);
+	struct page *page;
+
+	page = alloc_page(PGALLOC_GFP);
+	if (!page)
+		return NULL;
+	if (!pgtable_pmd_page_ctor(page)) {
+		__free_page(page);
+		return NULL;
+	}
+	return page_address(page);
 }
 
 static inline void pmd_free(struct mm_struct *mm, pmd_t *pmdp)
 {
 	BUG_ON((unsigned long)pmdp & (PAGE_SIZE-1));
+	pgtable_pmd_page_dtor(virt_to_page(pmdp));
 	free_page((unsigned long)pmdp);
 }

There is just one problem here. ARM KVM's stage2_pmd_free() calls into pmd_free() on a page
originally allocated with __get_free_page() and never went through pgtable_pmd_page_ctor().
So when ARCH_ENABLE_SPLIT_PMD_PTLOCK is enabled

stage2_pmd_free()
	pgtable_pmd_page_dtor()
		ptlock_free()
			kmem_cache_free(page_ptl_cachep, page->ptl)

Though SLUB implementation for kmem_cache_free() seems to be handling NULL page->ptl (as the
page never got it's lock allocated or initialized) correctly I am not sure if it is a right
thing to do.

Thanks for reminding me. This should be fixed as well. Will do it
in a separate patch.

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