mm/damon/core: do non-safe region walk on kdamond_apply_schemes()

From: SeongJae Park <[email protected]>

kdamond_apply_schemes() is using damon_for_each_region_safe(), which is safe for deallocation of the region inside the loop. However, the loop internal logic does not deallocate regions. Hence it is only wasting the next pointer. Also, it causes a problem.

When an address filter is applied, and there is a region that intersects with the filter, the filter splits the region on the filter boundary. The intention is to let DAMOS apply action to only filtered-in address ranges. However, it is using damon_for_each_region_safe(), which sets the next region before the execution of the iteration. Hence, the region that split and now will be next to the previous region, is simply ignored. As a result, DAMOS applies the action to target regions bit slower than expected, when the address filter is used. Shouldn't be a big problem but definitely better to be fixed. damos_skip_charged_region() was working around the issue using a double pointer hack.

Use damon_for_each_region(), which is safe for this use case. And drop the work around in damos_skip_charged_region().

---
This patch is required to fix a bug that is introduced by commit 500c9259f5a6, which is a backport of upstream commit b3723b596b54 ("mm/damon/core: fix unconditionally skip last region"). The backport allowed trying damon_split_region_at() of last region in damon_skip_charged_region(). If damon_split_region() fails, following line in damon_skip_charged_region() assigns wrong region pointer to the variable 'r', and propagate that via 'rp' double pointer. As a result, silent memory corruption can happen. damon_split_region_at() will fail only under extremem memory pressure, but still theoretically could happen. This backport fix it by removing the damon_next_region() call.

 mm/damon/core.c | 24 ++++++++++++------------
 1 file changed, 12 insertions(+), 12 deletions(-)

index c0dcb190b06e..8217815de898 100644
@@ -1734,16 +1734,18 @@ static bool damos_valid_target(struct damon_ctx *c, struct damon_target *t,
  * This function checks if a given region should be skipped or not for the
  * reason.  If only the starting part of the region has previously charged,
  * this function splits the region into two so that the second one covers the
- * area that not charged in the previous charge widnow and saves the second
- * region in *rp and returns false, so that the caller can apply DAMON action
- * to the second one.
+ * area that not charged in the previous charge widnow, and return true.  The
+ * caller can see the second one on the next iteration of the region walk.
+ * Note that this means the caller should use damon_for_each_region() instead
+ * of damon_for_each_region_safe().  If damon_for_each_region_safe() is used,
+ * the second region will just be ignored.
  *
- * Return: true if the region should be entirely skipped, false otherwise.
+ * Return: true if the region should be skipped, false otherwise.
  */
 static bool damos_skip_charged_region(struct damon_target *t,
-		struct damon_region **rp, struct damos *s, unsigned long min_sz_region)
+		struct damon_region *r, struct damos *s,
+		unsigned long min_sz_region)
 {
-	struct damon_region *r = *rp;
 	struct damos_quota *quota = &s->quota;
 	unsigned long sz_to_skip;
 	bool skip = false;

@@ -1770,9 +1772,7 @@ static bool damos_skip_charged_region(struct damon_target *t,
 				sz_to_skip = min_sz_region;
 			}
 			damon_split_region_at(t, r, sz_to_skip);
-			r = damon_next_region(r);
-			*rp = r;
-			skip = false;
+			return true;
 		}
 	}
 out:

@@ -2034,7 +2034,7 @@ static void damon_do_apply_schemes(struct damon_ctx *c,
 		if (quota->esz && quota->charged_sz >= quota->esz)
 			continue;
 
-		if (damos_skip_charged_region(t, &r, s, c->min_sz_region))
+		if (damos_skip_charged_region(t, r, s, c->min_sz_region))
 			continue;
 
 		if (!damos_valid_target(c, t, r, s))

@@ -2306,7 +2306,7 @@ static void damos_adjust_quota(struct damon_ctx *c, struct damos *s)
 static void kdamond_apply_schemes(struct damon_ctx *c)
 {
 	struct damon_target *t;
-	struct damon_region *r, *next_r;
+	struct damon_region *r;
 	struct damos *s;
 	unsigned long sample_interval = c->attrs.sample_interval ?
 		c->attrs.sample_interval : 1;

@@ -2329,7 +2329,7 @@ static void kdamond_apply_schemes(struct damon_ctx *c)
 
 	mutex_lock(&c->walk_control_lock);
 	damon_for_each_target(t, c) {
-		damon_for_each_region_safe(r, next_r, t)
+		damon_for_each_region(r, t)
 			damon_do_apply_schemes(c, t, r);
 	}
 

2.47.3

TL;DR: This patch is wrong, please ignore. I will send v2.

# sashiko review suggestions
#
# 1. Consider reducing recipients.  Maybe the author,
#    maintainers, reviewers, and mailing list of the
#    direct subsystem and parent susystem mailing lists
#    could be a starting point.
# 2. Add short summary of your opinion at the beginning.
#    For example:
#
#      TL;DR: sashiko found an issue.  I will respin.
#      TL;DR: sashiko found no issue on this patch.
#
# Please DON'T FORGET removing this comment block before
# sending this!

Somehow Sashiko stopped sending review as reply, so manually sharing it.

Forwarding full sashiko review in a reply format with my inline comments below, for sharing details of my view and doing followup discussions via mails if needed.

Correct. It is my mistake during porting. I will fix this in the next version.

Good finding. The bug exist in mainline too, and doesn't cause critical crash or corruption. Hence I don't think this is a blocker of this patch. I will separately work on it. Please let me know if anyone has a different opinion.

# hkml [1] generated a draft of this mail.  You can regenerate
# this using below command:
#
#     hkml patch sashiko_dev --for_forwarding \
#             [email protected]
#
# [1] https://github.com/sjp38/hackermail

Thanks,
SJ

I will fix the state leak. The second one is a minor bug that also exist in mainline, so no blocker of this patch in my opinion. Please read my previous reply [1] for more details.

[1] https://lore.kernel.org/[email protected]

Thanks,
SJ

[...]