linux-kernel - Re: scsi: non atomic allocation in mempool

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Date:	Mon, 5 Jan 2015 20:32:13 +0100
From:	Christoph Hellwig <hch@....de>
To:	Jens Axboe <axboe@...nel.dk>
Cc:	Sasha Levin <sasha.levin@...cle.com>, bvanassche@....org,
	hare@...e.de, JBottomley@...allels.com, linux-scsi@...r.kernel.org,
	LKML <linux-kernel@...r.kernel.org>,
	Dave Jones <davej@...hat.com>
Subject: Re: scsi: non atomic allocation in mempool_alloc in atomic context

On Mon, Jan 05, 2015 at 12:00:58PM -0700, Jens Axboe wrote:
> That's not quite true, the only guarantee is that it WILL execute on the
> CPU (or CPUs) that are set in the mask. So unless it ends up offloading
> the run to a specific workqueue, we'll disable preempt in the current
> path before ->queue_rq() is called.

Oops.  Indeed, with those recent changes ->queue_rq can't safely block
for memory allocatios anymore.

The patch below should fix it:

---
From: Christoph Hellwig <hch@....de>
Subject: scsi: ->queue_rq can't sleep

Since Linux 3.19 blk-mq may disable preemption before calling into
->queue_rq, so we can't actually sleep anymore.

Signed-off-by: Christoph Hellwig <hch@....de>
---
 drivers/scsi/scsi_lib.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
index 9ea95dd..6d5c0b8 100644
--- a/drivers/scsi/scsi_lib.c
+++ b/drivers/scsi/scsi_lib.c
@@ -591,7 +591,6 @@ static void scsi_free_sgtable(struct scsi_data_buffer *sdb, bool mq)
 static int scsi_alloc_sgtable(struct scsi_data_buffer *sdb, int nents, bool mq)
 {
 	struct scatterlist *first_chunk = NULL;
-	gfp_t gfp_mask = mq ? GFP_NOIO : GFP_ATOMIC;
 	int ret;
 
 	BUG_ON(!nents);
@@ -606,7 +605,7 @@ static int scsi_alloc_sgtable(struct scsi_data_buffer *sdb, int nents, bool mq)
 	}
 
 	ret = __sg_alloc_table(&sdb->table, nents, SCSI_MAX_SG_SEGMENTS,
-			       first_chunk, gfp_mask, scsi_sg_alloc);
+			       first_chunk, GFP_ATOMIC, scsi_sg_alloc);
 	if (unlikely(ret))
 		scsi_free_sgtable(sdb, mq);
 	return ret;
-- 
1.9.1

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