Commit 13bd6914 authored Oct 17, 2020 by Jens Axboe

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: 1a0a7853 ("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>

parent 58852d4d

mm/filemap.c

+8 −0

Original line number	Diff line number	Diff line
		@@ -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;

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