linux-kernel - [PATCH 6.6.y v2 1/1] block: Fix bounce check logic in blk_queue_may

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Message-Id: <20250725112710.219313-1-quic_hardshar@quicinc.com>
Date: Fri, 25 Jul 2025 16:57:10 +0530
From: Hardeep Sharma <quic_hardshar@...cinc.com>
To: Jens Axboe <axboe@...nel.dk>, Hannes Reinecke <hare@...e.de>,
        "Martin K . Petersen" <martin.petersen@...cle.com>
Cc: linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org, Hardeep Sharma <quic_hardshar@...cinc.com>
Subject: [PATCH 6.6.y v2 1/1] block: Fix bounce check logic in blk_queue_may_bounce()

Buffer bouncing is needed only when memory exists above the lowmem region,
i.e., when max_low_pfn < max_pfn. The previous check (max_low_pfn >=
max_pfn) was inverted and prevented bouncing when it could actually be
required.

Note that bouncing depends on CONFIG_HIGHMEM, which is typically enabled
on 32-bit ARM where not all memory is permanently mapped into the kernel’s
lowmem region.

Fixes: 9bb33f24abbd0 ("block: refactor the bounce buffering code")
Cc: stable@...r.kernel.org
Signed-off-by: Hardeep Sharma <quic_hardshar@...cinc.com>
---
Changelog v1..v2:

* Updated subject line

 block/blk.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/block/blk.h b/block/blk.h
index 67915b04b3c1..f8a1d64be5a2 100644
--- a/block/blk.h
+++ b/block/blk.h
@@ -383,7 +383,7 @@ static inline bool blk_queue_may_bounce(struct request_queue *q)
 {
 	return IS_ENABLED(CONFIG_BOUNCE) &&
 		q->limits.bounce == BLK_BOUNCE_HIGH &&
-		max_low_pfn >= max_pfn;
+		max_low_pfn < max_pfn;
 }
 
 static inline struct bio *blk_queue_bounce(struct bio *bio,
-- 
2.25.1