[PATCH 5.9 70/74] mm: mark async iocb read as NOWAIT once some data has been copied

Greg Kroah-Hartman
2020-10-31T11:46:01Z

From: Jens Axboe <axboe@kernel.dk>

commit 13bd691421bc191a402d2e0d3da5f248d170a632 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
mm/filemap.c | 8 ++++++++
1 file changed, 8 insertions(+)

--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2179,6 +2179,14 @@ ssize_t generic_file_buffered_read(struc
last_index = (*ppos + iter->count + PAGE_SIZE-1) >> PAGE_SHIFT;
offset = *ppos & ~PAGE_MASK;

+ /*
+ * 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 (written && (iocb->ki_flags & IOCB_WAITQ))
+ iocb->ki_flags |= IOCB_NOWAIT;
+
for (;;) {
struct page *page;
pgoff_t end_index;