Re: [RFC PATCH v1.1 1/8] mm/damon/core: safely validate src on damon_commit_ctx()

[SJ Park <sj@kernel.org>]

On Thu,  2 Jul 2026 14:35:58 -0700 SJ Park <sj@kernel.org> wrote:

> On Thu, 02 Jul 2026 21:21:42 +0000 sashiko-bot@kernel.org wrote:
[...]
> > [Severity: High]
> > This is a pre-existing issue, but does this unintentionally corrupt the running
> > quota state?
> > 
> > When __damon_commit_ctx() is called with the running context (dst) as the
> > source, it copies the schemes via damon_commit_schemes(), which calls
> > damon_new_scheme() and passes a pointer to the active scheme's quota:
> > 
> > mm/damon/core.c:damon_commit_schemes() {
> > 	...
> > 	new_scheme = damon_new_scheme(&src_scheme->pattern,
> > 				src_scheme->action,
> > 				src_scheme->apply_interval_us,
> > 				&src_scheme->quota, &src_scheme->wmarks,
> > 				NUMA_NO_NODE);
> > 	...
> > }
> > 
> > damon_new_scheme() then calls damos_quota_init() on that provided quota
> > pointer:
> > 
> > mm/damon/core.c:damon_new_scheme() {
> > 	...
> > 	scheme->quota = *(damos_quota_init(quota));
> > 	...
> > }
> > 
> > damos_quota_init() zeroes out the fields in-place:
> > 
> > mm/damon/core.c:damos_quota_init() {
> > 	...
> > 	quota->esz = 0;
> > 	quota->total_charged_sz = 0;
> > 	...
> > }
> > 
> > This mutates the running scheme's quota state, which would make the quota
> > effectively "full" mid-interval and prevent the scheme from applying its
> > actions until the next reset interval.
> 
> That doesn't matter because it is only testing purpose scheme, not the schemes
> of the running context.
> 
> > 
> > Since this patch moves this test context logic into the core
> > damon_commit_ctx(), doesn't this expose all API users like damon_reclaim
> > and damon_lru_sort to this state corruption when parameters are committed?
> 
> So, no problem.

I was misunderstanding Sashiko's point.  Karl is thankfully working [1] on
fixing this.

[1] https://lore.kernel.org/20260921003047.12041-2-kmehltretter@gmail.com


Thanks,
SJ

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