[RFC PATCH 2/4] mm/damon/core: set ctx->call_controls_obsolete in damon_new_ctx()

[SJ Park <sj@kernel.org>]

damon_ctx->call_controls_obsolete is used to disallow damon_call()
requests when the request cannot be served.  The field is unset and set
when the context execution is started and terminated, respectively.  The
intention is to allow damon_call() requests only while the context is
actively being executed.

damon_ctx constructor, damon_new_ctx() unsets the field, though.  As a
result, passing the damon_ctx parameter that never successfully
damon_start()-ed to damon_call() can indefinitely hang.  The callers
should ensure to avoid the case.  Such parameter validation is not
always simple.  Actually such bugs in DAMON_RECLAIM and DAMON_LRU_SORT
have been found and fixed [1].

Set the field in damon_new_ctx(), so that DAMON API callers can pass the
context parameter to damon_call() without the additional check.

[1] https://lore.kernel.org/20260803134646.16640-1-sj@kernel.org

Signed-off-by: SJ Park <sj@kernel.org>
---
 mm/damon/core.c | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index 5882f9c94c47f..5a92e4fac6d92 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -920,6 +920,7 @@ struct damon_ctx *damon_new_ctx(void)
 	INIT_LIST_HEAD(&ctx->adaptive_targets);
 	INIT_LIST_HEAD(&ctx->schemes);
 
+	ctx->call_controls_obsolete = true;
 	prandom_seed_state(&ctx->rnd_state, get_random_u64());
 
 	return ctx;
@@ -2178,10 +2179,6 @@ int damon_kdamond_pid(struct damon_ctx *ctx)
  * synchronization.  The return value of the function will be saved in
  * &damon_call_control->return_code.
  *
- * Note that this function should be called only after damon_start() with the
- * @ctx has succeeded.  Otherwise, this function could fall into an indefinite
- * wait.
- *
  * When this function is failed, the @ctx is guaranteed to be stopped.
  *
  * Return: 0 on success, negative error code otherwise.
-- 
2.47.3