From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: stable@vger.kernel.org
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	patches@lists.linux.dev,
	=?UTF-8?q?Christoph=20B=C3=B6hmwalder?= <christoph.boehmwalder@linbit.com>,
	Christoph Hellwig <hch@lst.de>,
	Jens Axboe <axboe@kernel.dk>,
	Sasha Levin <sashal@kernel.org>
Subject: [PATCH 6.18 497/641] drbd: always set BLK_FEAT_STABLE_WRITES
Date: Tue, 24 Feb 2026 17:23:43 -0800
Message-ID: <20260225012400.563785944@linuxfoundation.org>
In-Reply-To: <20260225012348.915798704@linuxfoundation.org>
References: <20260225012348.915798704@linuxfoundation.org>
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6.18-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Christoph Böhmwalder <christoph.boehmwalder@linbit.com>

[ Upstream commit 2ebc8d600fb907fa6b1e7095c0b6d84fc47e91ea ]

DRBD requires stable pages because it may read the same bio data
multiple times for local disk I/O and network transmission, and in
some cases for calculating checksums.

The BLK_FEAT_STABLE_WRITES flag is set when the device is first
created, but blk_set_stacking_limits() clears it whenever a
backing device is attached. In some cases the flag may be
inherited from the backing device, but we want it to be enabled
at all times.

Unconditionally re-enable BLK_FEAT_STABLE_WRITES in
drbd_reconsider_queue_parameters() after the queue parameter
negotiations.

Also, document why we want this flag enabled in the first place.

Fixes: 1a02f3a73f8c ("block: move the stable_writes flag to queue_limits")
Signed-off-by: Christoph Böhmwalder <christoph.boehmwalder@linbit.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 drivers/block/drbd/drbd_main.c |  3 ---
 drivers/block/drbd/drbd_nl.c   | 20 +++++++++++++++++++-
 2 files changed, 19 insertions(+), 4 deletions(-)

diff --git a/drivers/block/drbd/drbd_main.c b/drivers/block/drbd/drbd_main.c
index c73376886e7a5..1f6ac9202b66a 100644
--- a/drivers/block/drbd/drbd_main.c
+++ b/drivers/block/drbd/drbd_main.c
@@ -2659,9 +2659,6 @@ enum drbd_ret_code drbd_create_device(struct drbd_config_context *adm_ctx, unsig
 		 * connect.
 		 */
 		.max_hw_sectors		= DRBD_MAX_BIO_SIZE_SAFE >> 8,
-		.features		= BLK_FEAT_WRITE_CACHE | BLK_FEAT_FUA |
-					  BLK_FEAT_ROTATIONAL |
-					  BLK_FEAT_STABLE_WRITES,
 	};
 
 	device = minor_to_device(minor);
diff --git a/drivers/block/drbd/drbd_nl.c b/drivers/block/drbd/drbd_nl.c
index 91f3b8afb63ce..b502038be0a92 100644
--- a/drivers/block/drbd/drbd_nl.c
+++ b/drivers/block/drbd/drbd_nl.c
@@ -1296,6 +1296,8 @@ void drbd_reconsider_queue_parameters(struct drbd_device *device,
 		lim.max_segments = drbd_backing_dev_max_segments(device);
 	} else {
 		lim.max_segments = BLK_MAX_SEGMENTS;
+		lim.features = BLK_FEAT_WRITE_CACHE | BLK_FEAT_FUA |
+			       BLK_FEAT_ROTATIONAL | BLK_FEAT_STABLE_WRITES;
 	}
 
 	lim.max_hw_sectors = new >> SECTOR_SHIFT;
@@ -1318,8 +1320,24 @@ void drbd_reconsider_queue_parameters(struct drbd_device *device,
 		lim.max_hw_discard_sectors = 0;
 	}
 
-	if (bdev)
+	if (bdev) {
 		blk_stack_limits(&lim, &b->limits, 0);
+		/*
+		 * blk_set_stacking_limits() cleared the features, and
+		 * blk_stack_limits() may or may not have inherited
+		 * BLK_FEAT_STABLE_WRITES from the backing device.
+		 *
+		 * DRBD always requires stable writes because:
+		 * 1. The same bio data is read for both local disk I/O and
+		 *    network transmission. If the page changes mid-flight,
+		 *    the local and remote copies could diverge.
+		 * 2. When data integrity is enabled, DRBD calculates a
+		 *    checksum before sending the data. If the page changes
+		 *    between checksum calculation and transmission, the
+		 *    receiver will detect a checksum mismatch.
+		 */
+		lim.features |= BLK_FEAT_STABLE_WRITES;
+	}
 
 	/*
 	 * If we can handle "zeroes" efficiently on the protocol, we want to do
-- 
2.51.0