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

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Message-Id: <20201031113503.388678549@linuxfoundation.org>
Date:   Sat, 31 Oct 2020 12:36:52 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org,
        Kent Overstreet <kent.overstreet@...il.com>,
        Jens Axboe <axboe@...nel.dk>
Subject: [PATCH 5.9 70/74] mm: mark async iocb read as NOWAIT once some data has been copied

From: Jens Axboe <axboe@...nel.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@...r.kernel.org # v5.9
Fixes: 1a0a7853b901 ("mm: support async buffered reads in generic_file_buffered_read()")
Reported-by: Kent Overstreet <kent.overstreet@...il.com>
Signed-off-by: Jens Axboe <axboe@...nel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.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;