[PATCH 1/2] loop: force GFP_NOIO for underlying file systems allocations

[Christoph Hellwig]

File systems can and often do allocate memory in the read-write path.
If these allocations are done with __GFP_IO or __GFP_FS set they can
recurse into the file system or swap device on top of the loop device
and cause deadlocks.  Prevent this by forcing a noio scope over the
calls into the file system.

Signed-off-by: Christoph Hellwig <hch at lst.de>
---
 drivers/block/loop.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 1ec7417c7f00..71eccc5cfffb 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -1905,6 +1905,15 @@ static void loop_handle_cmd(struct loop_cmd *cmd)
 	int ret = 0;
 	struct mem_cgroup *old_memcg = NULL;
 	const bool use_aio = cmd->use_aio;
+	unsigned int memflags;
+
+	/*
+	 * We're calling into file system which could do be doing memory
+	 * allocations.  Ensure the memory reclaim does not cause I/O,
+	 * because that could end up in the user of this loop devices again and
+	 * deadlock.
+	 */
+	memflags = memalloc_noio_save();
 
 	if (write && (lo->lo_flags & LO_FLAGS_READ_ONLY)) {
 		ret = -EIO;
@@ -1942,6 +1951,7 @@ static void loop_handle_cmd(struct loop_cmd *cmd)
 		if (likely(!blk_should_fake_timeout(rq->q)))
 			blk_mq_complete_request(rq);
 	}
+	memalloc_noio_restore(memflags);
 }
 
 static void loop_process_work(struct loop_worker *worker,
-- 
2.45.2