Re: [PATCH 6.18.y] mm/damon/core: disallow time-quota setting zero esz

Re: [PATCH 6.18.y] mm/damon/core: disallow time-quota setting zero esz

[SeongJae Park]

TL; DR: This patch is broken, please drop.  I will send v2 soon, as a reply to
this.  Please read below for details.

On Mon, 04 May 2026 17:06:06 +0000 sashiko-bot@kernel.org wrote:

> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
> - [Medium] The newly added `ctx` parameter in `damos_set_effective_quota()` is unused, and the function incorrectly uses the `DAMON_MIN_REGION` macro instead of `ctx->min_sz_region`.
> --
> 
> commit 8afd735e4c663dcbcca090e71f37ad10c37c705a
> Author: SeongJae Park <sj@kernel.org>
> 
> mm/damon/core: disallow time-quota setting zero esz
> 
> When DAMOS scheme throughput is very slow, the time quota can make the
> effective size quota smaller than the context's minimum region size, causing
> actions to be skipped. This commit attempts to fix the problem by ensuring
> the effective size quota is at least the minimum region size.
> 
> Link: https://lore.kernel.org/20260407003153.79589-1-sj@kernel.org
> Link: https://lore.kernel.org/20260405192504.110014-1-sj@kernel.org [1]
> 
> > diff --git a/mm/damon/core.c b/mm/damon/core.c
> > --- a/mm/damon/core.c
> > +++ b/mm/damon/core.c
> > @@ -2124,7 +2124,8 @@ static unsigned long damos_quota_score(struct damos_quota *quota)
> >  /*
> >   * Called only if quota->ms, or quota->sz are set, or quota->goals is not empty
> >   */
> > -static void damos_set_effective_quota(struct damos_quota *quota)
> > +static void damos_set_effective_quota(struct damos_quota *quota,
> > +		struct damon_ctx *ctx)
> >  {
> >  	unsigned long throughput;
> >  	unsigned long esz = ULONG_MAX;
> 
> [ ... ]
> 
> > @@ -2150,6 +2151,7 @@ static void damos_set_effective_quota(struct damos_quota *quota)
> >  		else
> >  			throughput = PAGE_SIZE * 1024;
> >  		esz = min(throughput * quota->ms, esz);
> > +		esz = max(DAMON_MIN_REGION, esz);
> 
> Should this be using ctx->min_sz_region instead of DAMON_MIN_REGION?
> 
> The commit message mentions bounding the effective size quota to
> damon_ctx->min_region_sz, and the ctx parameter was successfully added
> to the function signature. 
> 
> However, the ctx parameter is left unused here and the macro is used as a
> hardcoded bound instead. Since ctx->min_sz_region can be dynamically
> configured, can the effective size quota still fall below it with this
> implementation?

Correct.  Cherry-picking the original commit was causing build issue since the
upstream commit is renamed field (min_region_sz).  When fixing the build issue
I mistakenly assumed the field itslef is not yet introduced to 6.18 and using
DAMON_MIN_REGION.  It should use the field that not yet renamed
(min_sz_region), as Sashiko pointed out.

Greg, please drop this.  I will send a v2 soon, as a reply to this mail.

> 
> >  	}
> >  
> >  	if (quota->sz && quota->sz < esz)
> 
> -- 
> Sashiko AI review · https://sashiko.dev/#/patchset/20260504130127.25364-1-sj@kernel.org?part=1
> 


Thanks,
SJ

[PATCH 6.18.y v2] mm/damon/core: disallow time-quota setting zero esz

[SeongJae Park]

When the throughput of a DAMOS scheme is very slow, DAMOS time quota can
make the effective size quota smaller than damon_ctx->min_region_sz.  In
the case, damos_apply_scheme() will skip applying the action, because the
action is tried at region level, which requires >=min_region_sz size.
That is, the quota is effectively exceeded for the quota charge window.

Because no action will be applied, the total_charged_sz and
total_charged_ns are also not updated.  damos_set_effective_quota() will
try to update the effective size quota before starting the next charge
window.  However, because the total_charged_sz and total_charged_ns have
not updated, the throughput and effective size quota are also not changed.
Since effective size quota can only be decreased, other effective size
quota update factors including DAMOS quota goals and size quota cannot
make any change, either.

As a result, the scheme is unexpectedly deactivated until the user notices
and mitigates the situation.  The users can mitigate this situation by
changing the time quota online or re-install the scheme.  While the
mitigation is somewhat straightforward, finding the situation would be
challenging, because DAMON is not providing good observabilities for that.
Even if such observability is provided, doing the additional monitoring
and the mitigation is somewhat cumbersome and not aligned to the intention
of the time quota.  The time quota was intended to help reduce the user's
administration overhead.

Fix the problem by setting time quota-modified effective size quota be at
least min_region_sz always.

The issue was discovered [1] by sashiko.

Link: https://lore.kernel.org/20260407003153.79589-1-sj@kernel.org
Link: https://lore.kernel.org/20260405192504.110014-1-sj@kernel.org [1]
Fixes: 1cd243030059 ("mm/damon/schemes: implement time quota")
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: <stable@vger.kernel.org> # 5.16.x
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
(cherry picked from commit 8bbde987c2b84f80da0853f739f0a920386f8b99)
Signed-off-by: SeongJae Park <sj@kernel.org>
---
 mm/damon/core.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index 87b6c9c2d647..feec1b0c2bab 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -2124,7 +2124,8 @@ static unsigned long damos_quota_score(struct damos_quota *quota)
 /*
  * Called only if quota->ms, or quota->sz are set, or quota->goals is not empty
  */
-static void damos_set_effective_quota(struct damos_quota *quota)
+static void damos_set_effective_quota(struct damos_quota *quota,
+		struct damon_ctx *ctx)
 {
 	unsigned long throughput;
 	unsigned long esz = ULONG_MAX;
@@ -2150,6 +2151,7 @@ static void damos_set_effective_quota(struct damos_quota *quota)
 		else
 			throughput = PAGE_SIZE * 1024;
 		esz = min(throughput * quota->ms, esz);
+		esz = max(ctx->min_sz_region, esz);
 	}
 
 	if (quota->sz && quota->sz < esz)
@@ -2186,7 +2188,7 @@ static void damos_adjust_quota(struct damon_ctx *c, struct damos *s)
 	/* First charge window */
 	if (!quota->total_charged_sz && !quota->charged_from) {
 		quota->charged_from = jiffies;
-		damos_set_effective_quota(quota);
+		damos_set_effective_quota(quota, c);
 	}
 
 	/* New charge window starts */
@@ -2199,7 +2201,7 @@ static void damos_adjust_quota(struct damon_ctx *c, struct damos *s)
 		quota->charged_sz = 0;
 		if (trace_damos_esz_enabled())
 			cached_esz = quota->esz;
-		damos_set_effective_quota(quota);
+		damos_set_effective_quota(quota, c);
 		if (trace_damos_esz_enabled() && quota->esz != cached_esz)
 			damos_trace_esz(c, s, quota);
 	}
-- 
2.47.3