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

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

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.

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.

> > 
> > 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.

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.

Best regards,
Rui Yan