+ mm-damon-sysfs-remove-requested-targets-when-online-commit-inputs-fix.patch added to mm-hotfixes-unstable branch

+ mm-damon-sysfs-remove-requested-targets-when-online-commit-inputs-fix.patch added to mm-hotfixes-unstable branch

[Andrew Morton]

The patch titled
     Subject: mm/damon/sysfs: fix unnecessary monitoring results removal when online-commit inputs
has been added to the -mm mm-hotfixes-unstable branch.  Its filename is
     mm-damon-sysfs-remove-requested-targets-when-online-commit-inputs-fix.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-damon-sysfs-remove-requested-targets-when-online-commit-inputs-fix.patch

This patch will later appear in the mm-hotfixes-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: SeongJae Park <sj@kernel.org>
Subject: mm/damon/sysfs: fix unnecessary monitoring results removal when online-commit inputs
Date: Sat, 28 Oct 2023 21:33:53 +0000

Commit db27869df6ed ("mm/damon/sysfs: remove requested targets when
online-commit inputs") of mm-hotfixes-unstable tree[1] makes all targets
to removed and then added again based on the user input, for online-commit
inputs.  The commit says it is inefficient but ok as below:

    This could cause unnecessary memory dealloc and realloc operations,
    but this is not a hot code path.  Also, note that damon_target is
    stateless, and hence no data is lost

But that's not true.  'struct target' is containing the monitoring results
('->regions_list').  As a result, the patch makes all previous monitoring
results to be removed for every online-commit inputs.  Fix it by removing
targets only when really changed or removal is requested.

[1] https://lore.kernel.org/damon/20231022210735.46409-2-sj@kernel.org/

Link: https://lkml.kernel.org/r/20231028213353.45397-1-sj@kernel.org
Fixes: db27869df6ed ("mm/damon/sysfs: remove requested targets when online-commit inputs") # mm-hotfixes-unstable
Signed-off-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/damon/sysfs.c |   44 +++++++++++++++++++++++++++++++++++++++-----
 1 file changed, 39 insertions(+), 5 deletions(-)

--- a/mm/damon/sysfs.c~mm-damon-sysfs-remove-requested-targets-when-online-commit-inputs-fix
+++ a/mm/damon/sysfs.c
@@ -1150,23 +1150,57 @@ destroy_targets_out:
 	return err;
 }
 
+static int damon_sysfs_update_target(struct damon_target *target,
+		struct damon_ctx *ctx,
+		struct damon_sysfs_target *sys_target)
+{
+	struct pid *pid;
+	struct damon_region *r, *next;
+
+	if (!damon_target_has_pid(ctx))
+		return 0;
+
+	pid = find_get_pid(sys_target->pid);
+	if (!pid)
+		return -EINVAL;
+
+	/* no change to the target */
+	if (pid == target->pid) {
+		put_pid(pid);
+		return 0;
+	}
+
+	/* remove old monitoring results and update the target's pid */
+	damon_for_each_region_safe(r, next, target)
+		damon_destroy_region(r, target);
+	put_pid(target->pid);
+	target->pid = pid;
+	return 0;
+}
+
 static int damon_sysfs_set_targets(struct damon_ctx *ctx,
 		struct damon_sysfs_targets *sysfs_targets)
 {
 	struct damon_target *t, *next;
-	int i, err;
+	int i = 0, err;
 
 	/* Multiple physical address space monitoring targets makes no sense */
 	if (ctx->ops.id == DAMON_OPS_PADDR && sysfs_targets->nr > 1)
 		return -EINVAL;
 
 	damon_for_each_target_safe(t, next, ctx) {
-		if (damon_target_has_pid(ctx))
-			put_pid(t->pid);
-		damon_destroy_target(t);
+		if (i < sysfs_targets->nr) {
+			damon_sysfs_update_target(t, ctx,
+					sysfs_targets->targets_arr[i]);
+		} else {
+			if (damon_target_has_pid(ctx))
+				put_pid(t->pid);
+			damon_destroy_target(t);
+		}
+		i++;
 	}
 
-	for (i = 0; i < sysfs_targets->nr; i++) {
+	for (; i < sysfs_targets->nr; i++) {
 		struct damon_sysfs_target *st = sysfs_targets->targets_arr[i];
 
 		err = damon_sysfs_add_target(st, ctx);
_

Patches currently in -mm which might be from sj@kernel.org are

mm-damon-sysfs-remove-requested-targets-when-online-commit-inputs.patch
mm-damon-sysfs-remove-requested-targets-when-online-commit-inputs-fix.patch