Re: [RFC PATCH] mm/damon: fix damos quota walk-position tracking

[SJ Park <sj@kernel.org>]

On Wed, 26 Aug 2026 07:05:08 -0700 SJ Park <sj@kernel.org> wrote:

> On Wed, 26 Aug 2026 18:24:13 +0800 Liew Rui Yan <aethernet65535@gmail.com> wrote:
> 
> > On Tue, 25 Aug 2026 06:54:57 -0700 SJ Park <sj@kernel.org> wrote:
> > 
> > > On Tue, 25 Aug 2026 20:46:16 +0800 Liew Rui Yan <aethernet65535@gmail.com> wrote:
> > > 
> > > > DAMOS uses charge_target_from/charge_addr_from to remember how far a
> > > > quota-limited walk has progressed.  The current implementation has two
> > > > problems:
> > > > 
> > > > 1. Once set, the cursor unconditionally skips and resets at the last
> > > >    region of the tracked target, so the last region can be skipped even
> > > >    when it has not been processed.
> > > 
> > > I don't fully understand this.  Could you please clarify more?  Maybe adding a
> > > realistic example scenario would be helpful.
> > > 
> > 
> > Problem: Unconditional skip of the last region
> > 
> > In the current damos_skip_charged_region(), there is this logic:
> > 
> >     if (r == damon_last_region(t)) {
> >         quota->charge_target_from = NULL;
> >         quota->charge_addr_from = 0;
> >         return true; /* Skip */
> >     }
> > 
> > Scenario:
> > 1. Target has 2 regions: R1 (0-100 bytes) and R2 (100-200 bytes).
> > 
> > 2. Quota is configured to process only 50 bytes per window.
> > 
> > 3. Window 1: Processes R1 (0-50).  Quota is full.  Cursor is saved at
> >    (Target, 50).
> > 
> > 4. Window 2: Skips R1 (0-50).  Processes R1 (50-100).  Quota is full.
> >    Cursor is saved at (Target, 100), which is exactly the start of R2.
> > 
> > 5. Window 3: The loop reaches R2.  Because R2 is damon_last_region(t),
> >    the old code unconditionally returns true, skipping R2 entirely and
> >    resetting the cursor.
> > 
> > Result: R2 is permanently skipped even though it has never been
> > processed.
> 
> Ok, makes sense.  The user impact should be not that big, though.
> 
> > 
> > To fix this, the patch advances the cursor every time a region is
> > walked, regardless of whether it is applied or filtered out.  This
> > allows DAMON to accurately track whether the last region has already
> > been visited, eliminating the need for the unconditional reset.
> 
> Sounds like a big change compared to the problem.  Why we cannot modify the
> last region case?  Have you also considered other possible simpler approaches?

For example,

'''
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -2686,14 +2686,15 @@ static bool damos_skip_charged_region(struct damon_target *t,
        if (quota->charge_target_from) {
                if (t != quota->charge_target_from)
                        return true;
-               if (r == damon_last_region(t)) {
-                       quota->charge_target_from = NULL;
-                       quota->charge_addr_from = 0;
-                       return true;
-               }
                if (quota->charge_addr_from &&
-                               r->ar.end <= quota->charge_addr_from)
+                               r->ar.end <= quota->charge_addr_from) {
+                       if (r->ar.end == quota->charge_addr_from ||
+                                       r == damon_last_region(t)) {
+                               quota->charge_target_from = NULL;
+                               quota->charge_addr_from = 0;
+                       }
                        return true;
+               }

                if (quota->charge_addr_from && r->ar.start <
                                quota->charge_addr_from) {
'''

> 
> > 
> > > > 
> > > > 2. The cursor only advances when the quota becomes full.  Regions that
> > > >    are filtered out do not move the cursor, and the scheme can remain
> > > >    stuck on the same regions.
> > > 
> > > I don't fully understand this, either.  Could you pleae clarify more?
> > > 
> > 
> > Problem 2 is a false positive.  As long as the quota is not full, DAMON
> > will continue iterating to find applicable regions.  I mistakenly
> > assumed in the commit message that encountering filtered-out or invalid
> > regions would cause the cursor to stall.
> 
> Thank you for clarifying.  Please try to reduce this kind of false positives
> from the next time.
> 
> > 
> > This patch ensures that every target is traversed sequentially and
> > deterministically, even when the quota is set very low.  I omitted this
> > benefit in the initial problem description.  If you think it is okay, I
> > will add it in the next revision.
> 
> What's the problem and benefit?  I still don't get it.  More clarification
> would be nice.
> 
> 
> Thanks,
> SJ
> 
> [...]


Thanks,
SJ