[RESEND for-5.20] io_uring/net: fix overexcessive retries

[Pavel Begunkov <asml.silence@gmail.com>]

Length parameter of io_sg_from_iter() can be smaller than the iterator's
size, as it's with TCP, so when we set from->count at the end of the
function we truncate the iterator forcing TCP to return preliminary with
a short send. It affects zerocopy sends with large payload sizes and
leads to retries and possible request failures.

Fixes: 3ff1a0d395c00 ("io_uring: enable managed frags with register buffers")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
---
 io_uring/net.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/io_uring/net.c b/io_uring/net.c
index 0af8a02df580..7a5468cc905e 100644
--- a/io_uring/net.c
+++ b/io_uring/net.c
@@ -956,7 +956,7 @@ static int io_sg_from_iter(struct sock *sk, struct sk_buff *skb,
 	shinfo->nr_frags = frag;
 	from->bvec += bi.bi_idx;
 	from->nr_segs -= bi.bi_idx;
-	from->count = bi.bi_size;
+	from->count -= copied;
 	from->iov_offset = bi.bi_bvec_done;
 
 	skb->data_len += copied;
-- 
2.37.2