[RFC PATCH v2 1/9] mm/damon/vaddr: support page fault access check primitive

[Ravi Jonnalagadda]

The page-fault sampling primitive was implemented for the physical
address space only, so a virtual address space context had no way to
enable it.

Implement it for the virtual address space.  The prep installs the marker
whose fault reports the access on the sampling address of each region,
mirroring the physical address space primitive, and prepare_access_checks()
now dispatches on the enabled primitive as the physical address space one
does.

The target mm and the vma of the sampling address are both available
here, so the marker is installed on the mapping directly.  The physical
address space primitive has only a physical address and reaches its
mappings through a reverse mapping walk.

Signed-off-by: Ravi Jonnalagadda <ravis.opensrc@xxxxxxxxx>
---
 mm/damon/vaddr.c | 65 +++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 64 insertions(+), 1 deletion(-)

diff --git a/mm/damon/vaddr.c b/mm/damon/vaddr.c
index d5dde97b3cd0d..1f2d923e569d6 100644
--- a/mm/damon/vaddr.c
+++ b/mm/damon/vaddr.c
@@ -13,6 +13,7 @@
 #include <linux/pagemap.h>
 #include <linux/pagewalk.h>
 #include <linux/sched/mm.h>
+#include <asm/tlb.h>
 
 #include "../internal.h"
 #include "ops-common.h"
@@ -329,7 +330,8 @@ static void __damon_va_prepare_access_check(struct mm_struct *mm,
 	damon_va_mkold(mm, r->sampling_addr);
 }
 
-static void damon_va_prepare_access_checks(struct damon_ctx *ctx)
+/* Use page table accessed bits */
+static void damon_va_prepare_access_checks_abit(struct damon_ctx *ctx)
 {
 	struct damon_target *t;
 	struct mm_struct *mm;
@@ -345,6 +347,67 @@ static void damon_va_prepare_access_checks(struct damon_ctx *ctx)
 	}
 }
 
+/*
+ * Install the page table marker whose fault reports the access.  The target
+ * mm and the vma of the address are available here, so the marker is set on
+ * the mapping directly, without a reverse mapping walk.
+ */
+static void damon_va_change_protection(struct mm_struct *mm,
+		unsigned long addr)
+{
+	struct vm_area_struct *vma;
+	struct mmu_gather tlb;
+
+	/*
+	 * The sampling address is a random offset within the region, so round
+	 * it down to the page it falls in.  change_protection() walks whole
+	 * page table entries and expects a page aligned range.
+	 */
+	addr = ALIGN_DOWN(addr, PAGE_SIZE);
+
+	mmap_read_lock(mm);
+
+	vma = vma_lookup(mm, addr);
+	if (!vma || !vma_is_accessible(vma) || (vma->vm_flags & VM_PFNMAP))
+		goto unlock;
+
+	tlb_gather_mmu(&tlb, mm);
+	/* todo: batch or remove tlb flushing */
+	change_protection(&tlb, vma, addr, addr + PAGE_SIZE, MM_CP_DAMON);
+	tlb_finish_mmu(&tlb);
+
+unlock:
+	mmap_read_unlock(mm);
+}
+
+/* Use page faults */
+static void damon_va_prepare_access_checks_faults(struct damon_ctx *ctx)
+{
+	struct damon_target *t;
+	struct mm_struct *mm;
+	struct damon_region *r;
+
+	damon_for_each_target(t, ctx) {
+		mm = damon_get_mm(t);
+		if (!mm)
+			continue;
+		damon_for_each_region(r, t) {
+			r->sampling_addr = damon_rand(ctx, r->ar.start,
+					r->ar.end);
+			damon_va_change_protection(mm, r->sampling_addr);
+		}
+		mmput(mm);
+	}
+}
+
+static void damon_va_prepare_access_checks(struct damon_ctx *ctx)
+{
+	if (ctx->sample_control.primitives_enabled.page_table)
+		damon_va_prepare_access_checks_abit(ctx);
+	if (ctx->sample_control.primitives_enabled.page_fault)
+		damon_va_prepare_access_checks_faults(ctx);
+}
+
 struct damon_young_walk_private {
 	bool young;
 };
-- 
2.43.0