bcachefs: Thread with file documentation

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/thread_with_file.c

@@ -76,16 +76,16 @@ static bool stdio_redirect_has_output(struct stdio_redirect *stdio)
 	return stdio->output.buf.nr || stdio->done;
 }
 
-#define WRITE_BUFFER		4096
+#define STDIO_REDIRECT_BUFSIZE		4096
 
 static bool stdio_redirect_has_input_space(struct stdio_redirect *stdio)
 {
-	return stdio->input.buf.nr < WRITE_BUFFER || stdio->done;
+	return stdio->input.buf.nr < STDIO_REDIRECT_BUFSIZE || stdio->done;
 }
 
 static bool stdio_redirect_has_output_space(struct stdio_redirect *stdio)
 {
-	return stdio->output.buf.nr < WRITE_BUFFER || stdio->done;
+	return stdio->output.buf.nr < STDIO_REDIRECT_BUFSIZE || stdio->done;
 }
 
 static void stdio_buf_init(struct stdio_buf *buf)
@@ -171,11 +171,12 @@ static ssize_t thread_with_stdio_write(struct file *file, const char __user *ubu
 		}
 
 		spin_lock(&buf->lock);
-		if (buf->buf.nr < WRITE_BUFFER)
-			darray_make_room_gfp(&buf->buf, min(b, WRITE_BUFFER - buf->buf.nr), __GFP_NOWARN);
+		if (buf->buf.nr < STDIO_REDIRECT_BUFSIZE)
+			darray_make_room_gfp(&buf->buf,
+				min(b, STDIO_REDIRECT_BUFSIZE - buf->buf.nr), GFP_NOWAIT);
 		b = min(len, darray_room(buf->buf));
 
-		if (b && !copy_from_user_nofault(&buf->buf.data[buf->buf.nr], ubuf, b)) {
+		if (b && !copy_from_user_nofault(&darray_top(buf->buf), ubuf, b)) {
 			buf->buf.nr += b;
 			ubuf	+= b;
 			len	-= b;
@@ -338,7 +339,7 @@ void bch2_stdio_redirect_vprintf(struct stdio_redirect *stdio, bool nonblocking,
 		return;
 
 	spin_lock_irqsave(&buf->lock, flags);
-	bch2_darray_vprintf(&buf->buf, nonblocking ? __GFP_NOWARN : GFP_KERNEL, fmt, args);
+	bch2_darray_vprintf(&buf->buf, nonblocking ? GFP_NOWAIT : GFP_KERNEL, fmt, args);
 	spin_unlock_irqrestore(&buf->lock, flags);
 
 	wake_up(&buf->wait);

fs/bcachefs/thread_with_file.h

@@ -4,6 +4,38 @@
 
 #include "thread_with_file_types.h"
 
+/*
+ * Thread with file: Run a kthread and connect it to a file descriptor, so that
+ * it can be interacted with via fd read/write methods and closing the file
+ * descriptor stops the kthread.
+ *
+ * We have two different APIs:
+ *
+ * thread_with_file, the low level version.
+ * You get to define the full file_operations, including your release function,
+ * which means that you must call bch2_thread_with_file_exit() from your
+ * .release method
+ *
+ * thread_with_stdio, the higher level version
+ * This implements full piping of input and output, including .poll.
+ *
+ * Notes on behaviour:
+ *  - kthread shutdown behaves like writing or reading from a pipe that has been
+ *    closed
+ *  - Input and output buffers are 4096 bytes, although buffers may in some
+ *    situations slightly exceed that limit so as to avoid chopping off a
+ *    message in the middle in nonblocking mode.
+ *  - Input/output buffers are lazily allocated, with GFP_NOWAIT allocations -
+ *    should be fine but might change in future revisions.
+ *  - Output buffer may grow past 4096 bytes to deal with messages that are
+ *    bigger than 4096 bytes
+ *  - Writing may be done blocking or nonblocking; in nonblocking mode, we only
+ *    drop entire messages.
+ *
+ * To write, use stdio_redirect_printf()
+ * To read, use stdio_redirect_read() or stdio_redirect_readline()
+ */
+
 struct task_struct;
 
 struct thread_with_file {