mm/damon/core: document damos_commit_dests() failure semantics

Add a kernel-doc-like comment to damos_commit_dests() documenting its
allocation failure contract: on -ENOMEM, the destination structure is left
in a partially torn-down state that is safe to deallocate via
damon_destroy_scheme(), but must not be reused for further commits.

This was unclear from the code alone and led to a separate patch [1]
attempting to reset nr_dests on failure.  Make the intended usage explicit
so future readers do not repeat the confusion.

Link: https://lkml.kernel.org/r/20260320143648.91673-1-sj@kernel.org
Link: https://lore.kernel.org/20260318214939.36100-1-objecting@objecting.org
Signed-off-by: Josh Law <objecting@objecting.org>
Reviewed-by: SeongJae Park <sj@kernel.org>
Signed-off-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/damon/core.c b/mm/damon/core.c
index f342bee002dc8f2b1682e1a3ccfb2fee6a0ff9b8..db6c67e52d2b86a26e08e58f2e7cf6af64257eaf 100644 (file)
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -1060,6 +1060,23 @@ static void damos_set_filters_default_reject(struct damos *s)
                damos_filters_default_reject(&s->ops_filters);
 }
 
+/*
+ * damos_commit_dests() - Copy migration destinations from @src to @dst.
+ * @dst:       Destination structure to update.
+ * @src:       Source structure to copy from.
+ *
+ * If the number of destinations has changed, the old arrays in @dst are freed
+ * and new ones are allocated.  On success, @dst contains a full copy of
+ * @src's arrays and count.
+ *
+ * On allocation failure, @dst is left in a partially torn-down state: its
+ * arrays may be NULL and @nr_dests may not reflect the actual allocation
+ * sizes.  The structure remains safe to deallocate via damon_destroy_scheme(),
+ * but callers must not reuse @dst for further commits — it should be
+ * discarded.
+ *
+ * Return: 0 on success, -ENOMEM on allocation failure.
+ */
 static int damos_commit_dests(struct damos_migrate_dests *dst,
                struct damos_migrate_dests *src)
 {