archive mirror
 help / color / mirror / Atom feed
From: Jens Axboe <>
Cc: Jens Axboe <>
Subject: [PATCH 2/3] io_uring: punt short reads to async context
Date: Wed, 29 May 2019 14:29:47 -0600	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

We can encounter a short read when we're doing buffered reads and the
data is partially cached. Right now we just return the short read, but
that forces the application to read that CQE, then issue another SQE
to finish the read. That read will not be cached, and hence will result
in an async punt.

It's more efficient to do that async punt from within the kernel, as
that will the not need two round trips more to the kernel.

Signed-off-by: Jens Axboe <>
 fs/io_uring.c | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 23e08c10f486..92debd8be535 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -1089,7 +1089,7 @@ static int io_read(struct io_kiocb *req, const struct sqe_submit *s,
 	struct iov_iter iter;
 	struct file *file;
 	size_t iov_count;
-	ssize_t ret;
+	ssize_t read_size, ret;
 	ret = io_prep_rw(req, s, force_nonblock);
 	if (ret)
@@ -1105,13 +1105,24 @@ static int io_read(struct io_kiocb *req, const struct sqe_submit *s,
 	if (ret < 0)
 		return ret;
+	read_size = ret;
 	iov_count = iov_iter_count(&iter);
 	ret = rw_verify_area(READ, file, &kiocb->ki_pos, iov_count);
 	if (!ret) {
 		ssize_t ret2;
-		/* Catch -EAGAIN return for forced non-blocking submission */
 		ret2 = call_read_iter(file, kiocb, &iter);
+		/*
+		 * In case of a short read, punt to async. This can happen
+		 * if we have data partially cached. Alternatively we can
+		 * return the short read, in which case the application will
+		 * need to issue another SQE and wait for it. That SQE will
+		 * need async punt anyway, so it's more efficient to do it
+		 * here.
+		 */
+		if (force_nonblock && ret2 > 0 && ret2 < read_size)
+			ret2 = -EAGAIN;
+		/* Catch -EAGAIN return for forced non-blocking submission */
 		if (!force_nonblock || ret2 != -EAGAIN) {
 			io_rw_done(kiocb, ret2);
 		} else {

  parent reply	other threads:[~2019-05-29 20:29 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-05-29 20:29 [PATCHSET v2 0/3] io_uring: support for linked SQEs Jens Axboe
2019-05-29 20:29 ` [PATCH 1/3] uio: make import_iovec()/compat_import_iovec() return bytes on success Jens Axboe
2019-05-29 20:29 ` Jens Axboe [this message]
2019-05-29 20:29 ` [PATCH 3/3] io_uring: add support for sqe links Jens Axboe
  -- strict thread matches above, loose matches on Subject: below --
2019-05-17 21:41 [PATCHSET 0/3] io_uring: support for linked SQEs Jens Axboe
2019-05-17 21:41 ` [PATCH 2/3] io_uring: punt short reads to async context Jens Axboe

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \ \ \ \ \

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).