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Subject: Re: [PATCH v29 07/13] mm/damon: Implement a debugfs-based user space
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+ Returns non-empty string on success, negarive error code otherwise.

Nit: typo, 'negarive'


+    if (ctx->kdamond) {
+        if (targetid_is_pid(ctx))
+            dbgfs_put_pids(targets, nr_targets);
+        ret = -EBUSY;
+        goto unlock_out;

I don't understand this block, what does EBUSY mean in this context?

+    if (sscanf(kbuf, "%s", kbuf) != 1) {
+        kfree(kbuf);
+        return -EINVAL;
+    }

Nit, this diverts from other patterns use elsewhere where error 
conditions that need to free memory use a goto label.