[PATCH v3] LoongArch: Share the same pmd between vDSO space and stack

[Bibo Mao]

Currently vDSO virtual address space is randomed within range 64M
below TASK_SIZE, and next is randomed stack space within generic
range 8M. so vDSO space uses different pmd entry with stack.

With idea from x86, vDSO space can share the same pmd entry with
stack, only that randomed range is smaller than before. This will
save one PTE table page for every thread. And vDSO base is set as
even page aligned if there is enough VA space, so that less TLB entries
can be used for vDSO.

Also this patch removes ____cacheline_aligned_in_smp property for
vdso_pcpu_data structure, cache line aligned property is generally
used for frequently modified data like lock to avoid cache-thrash
on smp system. The struct element node is almost constant in
struct vdso_pcpu_data, so cache line aligned property is not
necessary, and it will use more memory.

Signed-off-by: Bibo Mao <maobibo@xxxxxxxxxxx>
---
Changes in v3:
 1. Code refined for vDSO addr calculation for even page 

Changes in v2:
 1. Set vDSO base address as even page address if possible, less TLB
entries may be used for vDSO since per TLB entry contains two pages.
 2. Add PF_RANDOMIZE flag checking to set random address for vDSO.

---
 arch/loongarch/include/asm/processor.h |  2 --
 arch/loongarch/include/asm/vdso/vdso.h |  2 +-
 arch/loongarch/kernel/process.c        |  4 ---
 arch/loongarch/kernel/vdso.c           | 50 +++++++++++++++++++++-----
 4 files changed, 43 insertions(+), 15 deletions(-)

diff --git a/arch/loongarch/include/asm/processor.h b/arch/loongarch/include/asm/processor.h
index c3bc44b5f5b3..5870b2785968 100644
--- a/arch/loongarch/include/asm/processor.h
+++ b/arch/loongarch/include/asm/processor.h
@@ -43,8 +43,6 @@
 
 #endif
 
-#define VDSO_RANDOMIZE_SIZE	(TASK_IS_32BIT_ADDR ? SZ_1M : SZ_64M)
-
 unsigned long stack_top(void);
 #define STACK_TOP stack_top()
 
diff --git a/arch/loongarch/include/asm/vdso/vdso.h b/arch/loongarch/include/asm/vdso/vdso.h
index 5a12309d9fb5..d57de1887bb2 100644
--- a/arch/loongarch/include/asm/vdso/vdso.h
+++ b/arch/loongarch/include/asm/vdso/vdso.h
@@ -12,7 +12,7 @@
 
 struct vdso_pcpu_data {
 	u32 node;
-} ____cacheline_aligned_in_smp;
+};
 
 struct loongarch_vdso_data {
 	struct vdso_pcpu_data pdata[NR_CPUS];
diff --git a/arch/loongarch/kernel/process.c b/arch/loongarch/kernel/process.c
index 767d94cce0de..059e52d59297 100644
--- a/arch/loongarch/kernel/process.c
+++ b/arch/loongarch/kernel/process.c
@@ -296,10 +296,6 @@ unsigned long stack_top(void)
 	top -= PAGE_ALIGN(current->thread.vdso->size);
 	top -= VVAR_SIZE;
 
-	/* Space to randomize the VDSO base */
-	if (current->flags & PF_RANDOMIZE)
-		top -= VDSO_RANDOMIZE_SIZE;
-
 	return top;
 }
 
diff --git a/arch/loongarch/kernel/vdso.c b/arch/loongarch/kernel/vdso.c
index 14941e4be66d..aa472fc6b128 100644
--- a/arch/loongarch/kernel/vdso.c
+++ b/arch/loongarch/kernel/vdso.c
@@ -148,16 +148,49 @@ int vdso_join_timens(struct task_struct *task, struct time_namespace *ns)
 }
 #endif
 
-static unsigned long vdso_base(void)
+/*
+ * Put the vdso above the (randomized) stack with another randomized
+ * offset.  This way there is no hole in the middle of address space.
+ * To save memory make sure it is still in the same PTE as the stack
+ * top.  This doesn't give that many random bits.
+ *
+ * Note that this algorithm is imperfect: the distribution of the vdso
+ * start address within a PMD is biased toward the end.
+ */
+static inline unsigned long vdso_base(unsigned long start, unsigned int len)
 {
-	unsigned long base = STACK_TOP;
+	unsigned long addr, end;
+	unsigned long offset;
 
-	if (current->flags & PF_RANDOMIZE) {
-		base += get_random_u32_below(VDSO_RANDOMIZE_SIZE);
-		base = PAGE_ALIGN(base);
-	}
+	/*
+	 * Round up the start address.  It can start out unaligned as a result
+	 * of stack start randomization.
+	 */
+	start = PAGE_ALIGN(start);
+
+	/* Round the lowest possible end address up to a PMD boundary. */
+	end = (start + len + PMD_SIZE - 1) & PMD_MASK;
+	if (end >= TASK_SIZE)
+		end = TASK_SIZE;
+	end -= len;
+
+	if (end > start) {
+		offset = 0;
+		if (current->flags & PF_RANDOMIZE)
+			offset = get_random_u32_below(end - start);
+		addr = PAGE_ALIGN_DOWN(start + offset);
+
+		/*
+		 * There are two pages per TLB entry on LoongArch system.
+		 * Set vDSO base address as even page so that there is no
+		 * invalid odd pair page, less TLB entries are used for vDSO
+		 */
+		if (addr & PAGE_SIZE)
+			addr += PAGE_SIZE;
+	} else
+		addr = start;
 
-	return base;
+	return addr;
 }
 
 int arch_setup_additional_pages(struct linux_binprm *bprm, int uses_interp)
@@ -177,7 +210,8 @@ int arch_setup_additional_pages(struct linux_binprm *bprm, int uses_interp)
 	 */
 	size = VVAR_SIZE + info->size;
 
-	data_addr = get_unmapped_area(NULL, vdso_base(), size, 0, 0);
+	data_addr = get_unmapped_area(NULL, vdso_base(mm->start_stack, size),
+					size, 0, 0);
 	if (IS_ERR_VALUE(data_addr)) {
 		ret = data_addr;
 		goto out;

base-commit: 7de25c855b63453826ef678420831f98331d85fd
-- 
2.39.3