Re: [PATCH] mm/damon/core: avoid use of half-online-committed context

[Andrew Morton]

On Thu, 19 Mar 2026 07:52:17 -0700 SeongJae Park <sj@xxxxxxxxxx> wrote:

> One major usage of damon_call() is online DAMON parameters update.  It
> is done by calling damon_commit_ctx() inside the damon_call() callback
> function.  damon_commit_ctx() can fail for two reasons: 1) invalid
> parameters and 2) internal memory allocation failures.  In case of
> failures, the damon_ctx that attempted to be updated (commit
> destination) can be partially updated (or, corrupted from a
> perspective), and therefore shouldn't be used anymore.  The function
> only ensures the damon_ctx object can safely deallocated using
> damon_destroy_ctx().
> 
> The API callers are, however, calling damon_commit_ctx() only after
> asserting the parameters are valid, to avoid damon_commit_ctx() fails
> due to invalid input parameters.  But it can still theoretically fail if
> the internal memory allocation fails.  In the case, DAMON may run with
> the partially updated damon_ctx.  This can result in unexpected
> behaviors including even NULL pointer dereference in case of
> damos_commit_dests() failure [1].  Such allocation failure is arguably
> too small to fail, so the real world impact would be rare.  But, given
> the bad consequence, this needs to be fixed.
> 
> Avoid such partially-committed (maybe-corrupted) damon_ctx use by saving
> the damon_commit_ctx() failure on the damon_ctx object.  For this,
> introduce damon_ctx->maybe_corrupted field.  damon_commit_ctx() sets it
> when it is failed.  kdamond_call() checks if the field is set after each
> damon_call_control->fn() is executed.  If it is set, ignore remaining
> callback requests and return.  All kdamond_call() callers including
> kdamond_fn() also check the maybe_corrupted field right after
> kdamond_call() invocations.  If the field is set, break the
> kdamond_fn() main loop so that DAMON sill doesn't use the context that
> might be corrupted.

I guess you saw the AI review?
	https://sashiko.dev/#/patchset/20260319145218.86197-1-sj%40kernel.org