[PATCH] mtd: block: prevent reclaim I/O during request processing

[Qingfang Deng]

The blktrans request callbacks run under dev->lock and may allocate
memory with GFP_KERNEL. For example, mtdblock_writesect() uses vmalloc()
to allocate its eraseblock cache. Direct reclaim can then recurse into
block I/O and deadlock on resources held by the request being processed.
syzbot reports a circular locking dependency involving the device mutex
and fs_reclaim.

Commit d5ba1c8ffd0b ("mtd: don't use PF_MEMALLOC") removed PF_MEMALLOC
from the MTD request thread. That flag had prevented direct reclaim,
so its removal made allocations during request processing eligible for
reclaim without excluding I/O.

Wrap request processing in mtd_queue_rq() in a NOIO scope. This covers
both the request and background callbacks, including allocations made
by lower MTD drivers, and prevents reclaim from initiating filesystem
or block I/O.

Fixes: d5ba1c8ffd0b ("mtd: don't use PF_MEMALLOC")
Assisted-by: Codex:gpt-6-astra
Reported-by: syzbot+e08e0a15269eefa87790@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=e08e0a15269eefa87790
Signed-off-by: Qingfang Deng <qingfang.deng@xxxxxxxxx>
---
 drivers/mtd/mtd_blkdevs.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/mtd/mtd_blkdevs.c b/drivers/mtd/mtd_blkdevs.c
index 4d2e7b7774e9..42f250344f0c 100644
--- a/drivers/mtd/mtd_blkdevs.c
+++ b/drivers/mtd/mtd_blkdevs.c
@@ -18,6 +18,7 @@
 #include <linux/spinlock.h>
 #include <linux/hdreg.h>
 #include <linux/mutex.h>
+#include <linux/sched/mm.h>
 #include <linux/uaccess.h>
 
 #include "mtdcore.h"
@@ -167,6 +168,7 @@ static blk_status_t mtd_queue_rq(struct blk_mq_hw_ctx *hctx,
 				 const struct blk_mq_queue_data *bd)
 {
 	struct mtd_blktrans_dev *dev;
+	unsigned int noio_flags;
 
 	dev = hctx->queue->queuedata;
 	if (!dev) {
@@ -174,10 +176,13 @@ static blk_status_t mtd_queue_rq(struct blk_mq_hw_ctx *hctx,
 		return BLK_STS_IOERR;
 	}
 
+	/* Reclaim must not recurse into I/O while processing requests. */
+	noio_flags = memalloc_noio_save();
 	spin_lock_irq(&dev->queue_lock);
 	list_add_tail(&bd->rq->queuelist, &dev->rq_list);
 	mtd_blktrans_work(dev);
 	spin_unlock_irq(&dev->queue_lock);
+	memalloc_noio_restore(noio_flags);
 
 	return BLK_STS_OK;
 }
-- 
2.43.0