[PATCH] mm: mark async iocb read as NOWAIT once some data has been, copied

[Jens Axboe <axboe@kernel.dk>]

Once we've copied some data for an iocb that is marked with IOCB_WAITQ,
we should no longer attempt to async lock a new page. Instead make sure
we return the copied amount, and let the caller retry, instead of
returning -EIOCBQUEUED for a new page.

This should only be possible with read-ahead disabled on the below
device, and multiple threads racing on the same file. Haven't been able
to reproduce on anything else.

Cc: stable@vger.kernel.org # v5.9
Fixes: 1a0a7853b901 ("mm: support async buffered reads in generic_file_buffered_read()")
Reported-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 mm/filemap.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index 1a6beaf69f49..029787fda56b 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2185,6 +2185,7 @@ ssize_t generic_file_buffered_read(struct kiocb *iocb,
 	pgoff_t index;
 	pgoff_t last_index;
 	pgoff_t prev_index;
+	ssize_t this_written = 0;
 	unsigned long offset;      /* offset into pagecache page */
 	unsigned int prev_offset;
 	int error = 0;
@@ -2207,6 +2208,14 @@ ssize_t generic_file_buffered_read(struct kiocb *iocb,
 
 		cond_resched();
 find_page:
+		/*
+		 * If we've already successfully copied some data, then we
+		 * can no longer safely return -EIOCBQUEUED. Hence mark
+		 * an async read NOWAIT at that point.
+		 */
+		if (this_written && (iocb->ki_flags & IOCB_WAITQ))
+			iocb->ki_flags |= IOCB_NOWAIT;
+
 		if (fatal_signal_pending(current)) {
 			error = -EINTR;
 			goto out;
@@ -2239,7 +2248,7 @@ ssize_t generic_file_buffered_read(struct kiocb *iocb,
 			 * serialisations and why it's safe.
 			 */
 			if (iocb->ki_flags & IOCB_WAITQ) {
-				if (written) {
+				if (this_written) {
 					put_page(page);
 					goto out;
 				}
@@ -2328,7 +2337,7 @@ ssize_t generic_file_buffered_read(struct kiocb *iocb,
 		prev_offset = offset;
 
 		put_page(page);
-		written += ret;
+		this_written += ret;
 		if (!iov_iter_count(iter))
 			goto out;
 		if (ret < nr) {
@@ -2448,6 +2457,7 @@ ssize_t generic_file_buffered_read(struct kiocb *iocb,
 
 	*ppos = ((loff_t)index << PAGE_SHIFT) + offset;
 	file_accessed(filp);
+	written += this_written;
 	return written ? written : error;
 }
 EXPORT_SYMBOL_GPL(generic_file_buffered_read);
-- 
2.28.0

-- 
Jens Axboe