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Subject[PATCH 08/17] pipe: Allow buffers to be marked read-whole-or-error for notifications [ver #5]
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Allow a buffer to be marked such that read() must return the entire buffer
in one go or return ENOBUFS. Multiple buffers can be amalgamated into a
single read, but a short read will occur if the next "whole" buffer won't
fit.

This is useful for watch queue notifications to make sure we don't split a
notification across multiple reads, especially given that we need to
fabricate an overrun record under some circumstances - and that isn't in
the buffers.

Signed-off-by: David Howells <dhowells@redhat.com>
---

fs/pipe.c | 8 +++++++-
include/linux/pipe_fs_i.h | 1 +
kernel/watch_queue.c | 2 +-
samples/watch_queue/watch_test.c | 2 +-
4 files changed, 10 insertions(+), 3 deletions(-)

diff --git a/fs/pipe.c b/fs/pipe.c
index 4d86274f9a6e..79cb0b5d83e1 100644
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -320,8 +320,14 @@ pipe_read(struct kiocb *iocb, struct iov_iter *to)
size_t written;
int error;

- if (chars > total_len)
+ if (chars > total_len) {
+ if (buf->flags & PIPE_BUF_FLAG_WHOLE) {
+ if (ret == 0)
+ ret = -ENOBUFS;
+ break;
+ }
chars = total_len;
+ }

error = pipe_buf_confirm(pipe, buf);
if (error) {
diff --git a/include/linux/pipe_fs_i.h b/include/linux/pipe_fs_i.h
index 1d3eaa233f4a..eaff59a2f074 100644
--- a/include/linux/pipe_fs_i.h
+++ b/include/linux/pipe_fs_i.h
@@ -8,6 +8,7 @@
#define PIPE_BUF_FLAG_ATOMIC 0x02 /* was atomically mapped */
#define PIPE_BUF_FLAG_GIFT 0x04 /* page is a gift */
#define PIPE_BUF_FLAG_PACKET 0x08 /* read() as a packet */
+#define PIPE_BUF_FLAG_WHOLE 0x10 /* read() must return entire buffer or error */

/**
* struct pipe_buffer - a linux kernel pipe buffer
diff --git a/kernel/watch_queue.c b/kernel/watch_queue.c
index c103e34f8705..ad64ea300f6d 100644
--- a/kernel/watch_queue.c
+++ b/kernel/watch_queue.c
@@ -115,7 +115,7 @@ static bool post_one_notification(struct watch_queue *wqueue,
buf->ops = &watch_queue_pipe_buf_ops;
buf->offset = offset;
buf->len = len;
- buf->flags = 0;
+ buf->flags = PIPE_BUF_FLAG_WHOLE;
pipe->head = head + 1;

if (!test_and_clear_bit(note, wqueue->notes_bitmap)) {
diff --git a/samples/watch_queue/watch_test.c b/samples/watch_queue/watch_test.c
index 4a311790d4ca..8628b4c5d567 100644
--- a/samples/watch_queue/watch_test.c
+++ b/samples/watch_queue/watch_test.c
@@ -63,7 +63,7 @@ static void saw_key_change(struct watch_notification *n, size_t len)
*/
static void consumer(int fd)
{
- unsigned char buffer[4096], *p, *end;
+ unsigned char buffer[433], *p, *end;
union {
struct watch_notification n;
unsigned char buf1[128];

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