linux-kernel - [PATCH] scsi: use GFP_NOFS to avoid circular locking dependency

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Message-ID: <20250128164700.6d8504c1@fangorn>
Date: Tue, 28 Jan 2025 16:47:00 -0500
From: Rik van Riel <riel@...riel.com>
To: "Martin K. Petersen" <martin.petersen@...cle.com>
Cc: Marc Aurèle La France <tsi@...oix.net>, "James E.J.
 Bottomley" <James.Bottomley@...senPartnership.com>,
 linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org,
 kernel-team@...a.com
Subject: [PATCH] scsi: use GFP_NOFS to avoid circular locking dependency

Filesystems can write to disk from page reclaim with filesystem
locks held, if __GFP_FS is set. Marc found a case where 
scsi_realloc_sdev_budget_map ends up in page reclaim with GFP_KERNEL, 
where it could try to take filesystem locks again, leading to a deadlock.

WARNING: possible circular locking dependency detected
6.13.0 #1 Not tainted
------------------------------------------------------
kswapd0/70 is trying to acquire lock:
ffff8881025d5d78 (&q->q_usage_counter(io)){++++}-{0:0}, at: blk_mq_submit_bio+0x461/0x6e0

but task is already holding lock:
ffffffff81ef5f40 (fs_reclaim){+.+.}-{0:0}, at: balance_pgdat+0x9f/0x760

The full lockdep splat can be found in Marc's report:

https://lkml.org/lkml/2025/1/24/1101

Avoid the potential deadlock by doing the allocation with GFP_NOFS.

Reported-by: Marc Aurèle La France <tsi@...oix.net>
Signed-off-by: Rik van Riel <riel@...riel.com>
---
 drivers/scsi/scsi_scan.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/scsi/scsi_scan.c b/drivers/scsi/scsi_scan.c
index f2093982b3db..93d6feef9c7c 100644
--- a/drivers/scsi/scsi_scan.c
+++ b/drivers/scsi/scsi_scan.c
@@ -245,7 +245,7 @@ static int scsi_realloc_sdev_budget_map(struct scsi_device *sdev,
 	}
 	ret = sbitmap_init_node(&sdev->budget_map,
 				scsi_device_max_queue_depth(sdev),
-				new_shift, GFP_KERNEL,
+				new_shift, GFP_NOFS,
 				sdev->request_queue->node, false, true);
 	if (!ret)
 		sbitmap_resize(&sdev->budget_map, depth);
-- 
2.47.1