Re: [PATCH 1/2] pipe: change pipe_write() to never add a zero-sized buffer

Oleg Nesterov
2025-02-10T17:22:40Z

Hi Prateek,

On 02/10, K Prateek Nayak wrote:
>
> 1-groups 1.00 [ -0.00]( 7.19) 0.95 [ 4.90](12.39)
> 2-groups 1.00 [ -0.00]( 3.54) 1.02 [ -1.92]( 6.55)
> 4-groups 1.00 [ -0.00]( 2.78) 1.01 [ -0.85]( 2.18)
> 8-groups 1.00 [ -0.00]( 1.04) 0.99 [ 0.63]( 0.77)
> 16-groups 1.00 [ -0.00]( 1.02) 1.00 [ -0.26]( 0.98)
>
> I don't see any regression / improvements from a performance standpoint

Yes, this patch shouldn't make any difference performance-wise, at least
in this case. Although I was thinking the same thing when I sent "pipe_read:
don't wake up the writer if the pipe is still full" ;)

> Tested-by: K Prateek Nayak <kprateek.nayak@amd.com>

Thanks! Please see v2, I've included you tag.

Any chance you can also test the patch below?

To me it looks like a cleanup which makes the "merge small writes" logic
more understandable. And note that "page-align the rest of the writes"
doesn't work anyway if "total_len & (PAGE_SIZE-1)" can't fit in the last
buffer.

However, in this particular case with DATASIZE=100 this patch can increase
the number of copy_page_from_iter()'s in pipe_write(). And with this change
receiver() can certainly get the short reads, so this can increase the
number of sys_read() calls.

So I am just curious if this change can cause any noticeable regression on
your machine.

Thank you!

Oleg.

--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -459,16 +459,16 @@ anon_pipe_write(struct kiocb *iocb, struct iov_iter *from)
was_empty = pipe_empty(head, pipe->tail);
chars = total_len & (PAGE_SIZE-1);
if (chars && !was_empty) {
- unsigned int mask = pipe->ring_size - 1;
- struct pipe_buffer *buf = &pipe->bufs[(head - 1) & mask];
+ struct pipe_buffer *buf = pipe_buf(pipe, head - 1);
int offset = buf->offset + buf->len;
+ int avail = PAGE_SIZE - offset;

- if ((buf->flags & PIPE_BUF_FLAG_CAN_MERGE) &&
- offset + chars <= PAGE_SIZE) {
+ if (avail && (buf->flags & PIPE_BUF_FLAG_CAN_MERGE)) {
ret = pipe_buf_confirm(pipe, buf);
if (ret)
goto out;

+ chars = min_t(ssize_t, chars, avail);
ret = copy_page_from_iter(buf->page, offset, chars, from);
if (unlikely(ret < chars)) {
ret = -EFAULT;