mm/damon/core: make damon_start() waits until kdamond_fn() starts

The cleanup tasks of kdamond threads including reset of corresponding
DAMON context's ->kdamond field and decrease of global nr_running_ctxs
counter is supposed to be executed by kdamond_fn().  However, commit
0f91d13366a4 ("mm/damon: simplify stop mechanism") made neither
damon_start() nor damon_stop() ensure the corresponding kdamond has
started the execution of kdamond_fn().

As a result, the cleanup can be skipped if damon_stop() is called fast
enough after the previous damon_start().  Especially the skipped reset
of ->kdamond could cause a use-after-free.

Fix it by waiting for start of kdamond_fn() execution from
damon_start().

Link: https://lkml.kernel.org/r/20231208175018.63880-1-sj@kernel.org
Fixes: 0f91d13366a4 ("mm/damon: simplify stop mechanism")
Signed-off-by: SeongJae Park <sj@kernel.org>
Reported-by: Jakub Acs <acsjakub@amazon.de>
Cc: Changbin Du <changbin.du@intel.com>
Cc: Jakub Acs <acsjakub@amazon.de>
Cc: <stable@vger.kernel.org> # 5.15.x
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/include/linux/damon.h b/include/linux/damon.h
index ab2f17d9926b599c2457f6e3dc834d12b6d8e7dc..e00ddf1ed39c05c945c0f31703d52f77881d615d 100644 (file)
--- a/include/linux/damon.h
+++ b/include/linux/damon.h
@@ -559,6 +559,8 @@ struct damon_ctx {
         * update
         */
        unsigned long next_ops_update_sis;
+       /* for waiting until the execution of the kdamond_fn is started */
+       struct completion kdamond_started;
 
 /* public: */
        struct task_struct *kdamond;

diff --git a/mm/damon/core.c b/mm/damon/core.c
index ce1562783e7e5df9941cd66e8eaf57db90f176de..3a05e71509b9db527e7938896dcde44521fe1735 100644 (file)
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -445,6 +445,8 @@ struct damon_ctx *damon_new_ctx(void)
        if (!ctx)
                return NULL;
 
+       init_completion(&ctx->kdamond_started);
+
        ctx->attrs.sample_interval = 5 * 1000;
        ctx->attrs.aggr_interval = 100 * 1000;
        ctx->attrs.ops_update_interval = 60 * 1000 * 1000;
@@ -668,11 +670,14 @@ static int __damon_start(struct damon_ctx *ctx)
        mutex_lock(&ctx->kdamond_lock);
        if (!ctx->kdamond) {
                err = 0;
+               reinit_completion(&ctx->kdamond_started);
                ctx->kdamond = kthread_run(kdamond_fn, ctx, "kdamond.%d",
                                nr_running_ctxs);
                if (IS_ERR(ctx->kdamond)) {
                        err = PTR_ERR(ctx->kdamond);
                        ctx->kdamond = NULL;
+               } else {
+                       wait_for_completion(&ctx->kdamond_started);
                }
        }
        mutex_unlock(&ctx->kdamond_lock);
@@ -1433,6 +1438,7 @@ static int kdamond_fn(void *data)
 
        pr_debug("kdamond (%d) starts\n", current->pid);
 
+       complete(&ctx->kdamond_started);
        kdamond_init_intervals_sis(ctx);
 
        if (ctx->ops.init)