mm: mark async iocb read as NOWAIT once some data has been copied

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>

diff --git a/mm/filemap.c b/mm/filemap.c
index 1a6beaf..e4101b5 100644 (file)
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2199,6 +2199,14 @@ ssize_t generic_file_buffered_read(struct kiocb *iocb,
        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;