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pipe: Allow buffers to be marked read-whole-or-error for notifications

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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>
This commit is contained in:
David Howells 2020-01-14 17:07:11 +00:00
parent f5b5a164f9
commit 8cfba76383
4 changed files with 10 additions and 3 deletions
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8
fs/pipe.c
View File

@ -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) {

1
include/linux/pipe_fs_i.h
View File

@ -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

2
kernel/watch_queue.c
View File

@ -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)) {

2
samples/watch_queue/watch_test.c
View File

@ -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];