[PATCH 2/2] io_uring: internally retry short reads

Jens Axboe
2020-08-13T17:58:13Z

We've had a few application cases of not handling short reads properly,
and it is understandable as short reads aren't really expected if the
application isn't doing non-blocking IO.

Now that we retain the iov_iter over retries, we can implement internal
retry pretty trivially. This ensures that we don't return a short read,
even for buffered reads on page cache conflicts.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
fs/io_uring.c | 72 ++++++++++++++++++++++++++++++++-------------------
1 file changed, 45 insertions(+), 27 deletions(-)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index a20fccf91d76..c6a9f1b11e85 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -509,6 +509,7 @@ struct io_async_msghdr {
struct io_async_rw {
struct iovec fast_iov[UIO_FASTIOV];
struct iov_iter iter;
+ size_t bytes_done;
struct wait_page_queue wpq;
};

@@ -914,7 +915,8 @@ static ssize_t io_import_iovec(int rw, struct io_kiocb *req,
struct iovec **iovec, struct iov_iter *iter,
bool needs_lock);
static int io_setup_async_rw(struct io_kiocb *req, struct iovec *iovec,
- struct iovec *fast_iov, struct iov_iter *iter);
+ struct iovec *fast_iov, struct iov_iter *iter,
+ bool force);

static struct kmem_cache *req_cachep;

@@ -2296,7 +2298,7 @@ static bool io_resubmit_prep(struct io_kiocb *req, int error)
ret = io_import_iovec(rw, req, &iovec, &iter, false);
if (ret < 0)
goto end_req;
- ret = io_setup_async_rw(req, iovec, inline_vecs, &iter);
+ ret = io_setup_async_rw(req, iovec, inline_vecs, &iter, false);
if (!ret)
return true;
kfree(iovec);
@@ -2586,6 +2588,14 @@ static void kiocb_done(struct kiocb *kiocb, ssize_t ret,
{
struct io_kiocb *req = container_of(kiocb, struct io_kiocb, rw.kiocb);

+ /* add previously done IO, if any */
+ if (req->io && req->io->rw.bytes_done > 0) {
+ if (ret < 0)
+ ret = req->io->rw.bytes_done;
+ else
+ ret += req->io->rw.bytes_done;
+ }
+
if (req->flags & REQ_F_CUR_POS)
req->file->f_pos = kiocb->ki_pos;
if (ret >= 0 && kiocb->ki_complete == io_complete_rw)
@@ -2932,6 +2942,7 @@ static void io_req_map_rw(struct io_kiocb *req, const struct iovec *iovec,
struct io_async_rw *rw = &req->io->rw;

memcpy(&rw->iter, iter, sizeof(*iter));
+ rw->bytes_done = 0;
if (!iovec) {
rw->iter.iov = rw->fast_iov;
if (rw->iter.iov != fast_iov)
@@ -2958,9 +2969,10 @@ static int io_alloc_async_ctx(struct io_kiocb *req)
}

static int io_setup_async_rw(struct io_kiocb *req, struct iovec *iovec,
- struct iovec *fast_iov, struct iov_iter *iter)
+ struct iovec *fast_iov, struct iov_iter *iter,
+ bool force)
{
- if (!io_op_defs[req->opcode].async_ctx)
+ if (!force && !io_op_defs[req->opcode].async_ctx)
return 0;
if (!req->io) {
if (__io_alloc_async_ctx(req))
@@ -3084,8 +3096,7 @@ static inline int kiocb_wait_page_queue_init(struct kiocb *kiocb,
* succeed, or in rare cases where it fails, we then fall back to using the
* async worker threads for a blocking retry.
*/
-static bool io_rw_should_retry(struct io_kiocb *req, struct iovec *iovec,
- struct iovec *fast_iov, struct iov_iter *iter)
+static bool io_rw_should_retry(struct io_kiocb *req)
{
struct kiocb *kiocb = &req->rw.kiocb;
int ret;
@@ -3094,8 +3105,8 @@ static bool io_rw_should_retry(struct io_kiocb *req, struct iovec *iovec,
if (req->flags & REQ_F_NOWAIT)
return false;

- /* already tried, or we're doing O_DIRECT */
- if (kiocb->ki_flags & (IOCB_DIRECT | IOCB_WAITQ))
+ /* Only for buffered IO */
+ if (kiocb->ki_flags & IOCB_DIRECT)
return false;
/*
* just use poll if we can, and don't attempt if the fs doesn't
@@ -3104,16 +3115,6 @@ static bool io_rw_should_retry(struct io_kiocb *req, struct iovec *iovec,
if (file_can_poll(req->file) || !(req->file->f_mode & FMODE_BUF_RASYNC))
return false;

- /*
- * If request type doesn't require req->io to defer in general,
- * we need to allocate it here
- */
- if (!req->io) {
- if (__io_alloc_async_ctx(req))
- return false;
- io_req_map_rw(req, iovec, fast_iov, iter);
- }
-
ret = kiocb_wait_page_queue_init(kiocb, &req->io->rw.wpq,
io_async_buf_func, req);
if (!ret) {
@@ -3167,29 +3168,46 @@ static int io_read(struct io_kiocb *req, bool force_nonblock,

ret2 = io_iter_do_read(req, iter);

- /* Catch -EAGAIN return for forced non-blocking submission */
- if (!force_nonblock || (ret2 != -EAGAIN && ret2 != -EIO)) {
- kiocb_done(kiocb, ret2, cs);
- } else {
+ if (!iov_iter_count(iter))
+ goto done;
+
+ if (ret2 >= 0) {
+ /* successful read, but partial */
+ if (req->flags & REQ_F_NOWAIT)
+ goto done;
+ io_size -= ret2;
copy_iov:
- ret = io_setup_async_rw(req, iovec, inline_vecs, iter);
+ ret = io_setup_async_rw(req, iovec, inline_vecs, iter, true);
if (ret)
goto out_free;
/* it's copied and will be cleaned with ->io */
iovec = NULL;
+ /* now use our persistent iterator, if we aren't already */
+ iter = &req->io->rw.iter;
+
+ if (ret2 > 0) {
+retry:
+ req->io->rw.bytes_done += ret2;
+ }
/* if we can retry, do so with the callbacks armed */
- if (io_rw_should_retry(req, iovec, inline_vecs, iter)) {
+ if (io_rw_should_retry(req)) {
ret2 = io_iter_do_read(req, iter);
if (ret2 == -EIOCBQUEUED) {
goto out_free;
+ } else if (ret2 == io_size) {
+ goto done;
} else if (ret2 != -EAGAIN) {
- kiocb_done(kiocb, ret2, cs);
- goto out_free;
+ /* we got some bytes, but not all. retry. */
+ if (ret2 > 0)
+ goto retry;
+ goto done;
}
}
kiocb->ki_flags &= ~IOCB_WAITQ;
return -EAGAIN;
}
+done:
+ kiocb_done(kiocb, ret2, cs);
out_free:
if (iovec)
kfree(iovec);
@@ -3282,7 +3300,7 @@ static int io_write(struct io_kiocb *req, bool force_nonblock,
kiocb_done(kiocb, ret2, cs);
} else {
copy_iov:
- ret = io_setup_async_rw(req, iovec, inline_vecs, iter);
+ ret = io_setup_async_rw(req, iovec, inline_vecs, iter, false);
if (ret)
goto out_free;
/* it's copied and will be cleaned with ->io */
--
2.28.0