mm/damon/vaddr: register a damon_operations for fixed virtual address ranges monitoring

Patch series "support fixed virtual address ranges monitoring".

The monitoring operations set for virtual address spaces automatically
updates the monitoring target regions to cover entire mappings of the
virtual address spaces as much as possible.  Some users could have more
information about their programs than kernel and therefore have interest
in not entire regions but only specific regions.  For such cases, the
automatic monitoring target regions updates are only unnecessary overhead
or distractions.

This patchset adds supports for the use case on DAMON's kernel API
(DAMON_OPS_FVADDR) and sysfs interface ('fvaddr' keyword for 'operations'
sysfs file).


This patch (of 3):

The monitoring operations set for virtual address spaces automatically
updates the monitoring target regions to cover entire mappings of the
virtual address spaces as much as possible.  Some users could have more
information about their programs than kernel and therefore have interest
in not entire regions but only specific regions.  For such cases, the
automatic monitoring target regions updates are only unnecessary overheads
or distractions.

For such cases, DAMON's API users can simply set the '->init()' and
'->update()' of the DAMON context's '->ops' NULL, and set the target
monitoring regions when creating the context.  But, that would be a dirty
hack.  Worse yet, the hack is unavailable for DAMON user space interface
users.

To support the use case in a clean way that can easily exported to the
user space, this commit adds another monitoring operations set called
'fvaddr', which is same to 'vaddr' but does not automatically update the
monitoring regions.  Instead, it will only respect the virtual address
regions which have explicitly passed at the initial context creation.

Note that this commit leave sysfs interface not supporting the feature
yet.  The support will be made in a following commit.

Link: https://lkml.kernel.org/r/20220426231750.48822-1-sj@kernel.org
Link: https://lkml.kernel.org/r/20220426231750.48822-2-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/damon.h

@@ -261,10 +261,13 @@ struct damos {
  * enum damon_ops_id - Identifier for each monitoring operations implementation
  *
  * @DAMON_OPS_VADDR:	Monitoring operations for virtual address spaces
+ * @DAMON_OPS_FVADDR:	Monitoring operations for only fixed ranges of virtual
+ *			address spaces
  * @DAMON_OPS_PADDR:	Monitoring operations for the physical address space
  */
 enum damon_ops_id {
 	DAMON_OPS_VADDR,
+	DAMON_OPS_FVADDR,
 	DAMON_OPS_PADDR,
 	NR_DAMON_OPS,
 };

mm/damon/sysfs.c

@@ -1694,6 +1694,7 @@ static struct kobj_type damon_sysfs_attrs_ktype = {
 /* This should match with enum damon_ops_id */
 static const char * const damon_sysfs_ops_strs[] = {
 	"vaddr",
+	"unsupported",	/* fvaddr is not supported by sysfs yet */
 	"paddr",
 };
 
@@ -1843,6 +1844,9 @@ static ssize_t operations_store(struct kobject *kobj,
 
 	for (id = 0; id < NR_DAMON_OPS; id++) {
 		if (sysfs_streq(buf, damon_sysfs_ops_strs[id])) {
+			/* fvaddr is not supported by sysfs yet */
+			if (id == DAMON_OPS_FVADDR)
+				return -EINVAL;
 			context->ops_id = id;
 			return count;
 		}

mm/damon/vaddr.c

@@ -753,8 +753,19 @@ static int __init damon_va_initcall(void)
 		.apply_scheme = damon_va_apply_scheme,
 		.get_scheme_score = damon_va_scheme_score,
 	};
+	/* ops for fixed virtual address ranges */
+	struct damon_operations ops_fvaddr = ops;
+	int err;
 
-	return damon_register_ops(&ops);
+	/* Don't set the monitoring target regions for the entire mapping */
+	ops_fvaddr.id = DAMON_OPS_FVADDR;
+	ops_fvaddr.init = NULL;
+	ops_fvaddr.update = NULL;
+
+	err = damon_register_ops(&ops);
+	if (err)
+		return err;
+	return damon_register_ops(&ops_fvaddr);
 };
 
 subsys_initcall(damon_va_initcall);