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

[Liew Rui Yan <aethernet65535@gmail.com>]

On Thu, 27 Aug 2026 17:40:48 -0700 SJ Park <sj@kernel.org> wrote:

> On Fri, 28 Aug 2026 02:08:22 +0800 Liew Rui Yan <aethernet65535@gmail.com> wrote:
> 
> > On Wed, 26 Aug 2026 17:44:38 -0700 SJ Park <sj@kernel.org> wrote:
> > 
> > > 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) {
> > > '''
> > > 
> > 
> > Thank you for the example!
> > 
> > While your approach works, I am curious, why should the cursor be reset
> > every time the function returns false (does not skip)?
> 
> It doesn't.  It resets charge_{target,addr}_from only once after the regions to
> skip are all skipped.  Am I missing something?

You are right.

My concern was that the current
'return false == reset charge_{target, addr}_from' might be a bit hard
to understand.  However, I realize that my change was quite significant.

To make the existing logic clearer for future readers, I think adding a
brief comment would be helpful.  For example:

'''
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -2368,6 +2368,15 @@ static bool damos_skip_charged_region(struct damon_target *t,
                        damon_split_region_at(t, r, sz_to_skip);
                        return true;
                }
+               /*
+                * Reset the charge_{target,addr}_from so that the remaining
+                * regions in this/next target can be processed normally.  If
+                * the quota becomes full later during the walk,
+                * damos_apply_scheme() will update the
+                * charge_{target,addr}_from to the correct position.
+                * Otherwise, it implies that all applicable regions in this
+                * target have been processed.
+                */
                quota->charge_target_from = NULL;
                quota->charge_addr_from = 0;
        }
'''

If this is not necessary or redundant, I am perfectly fine with dropping
it and just applying your minimal fix for the last-region issue in the
next revision.

Best regards,
Rui Yan