linux-kernel - [PATCH] md/md-linear: Enable atomic writes

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Message-ID: <20250903161052.3326176-1-john.g.garry@oracle.com>
Date: Wed,  3 Sep 2025 16:10:52 +0000
From: John Garry <john.g.garry@...cle.com>
To: song@...nel.org, yukuai3@...wei.com
Cc: linux-raid@...r.kernel.org, linux-kernel@...r.kernel.org,
        martin.petersen@...cle.com, John Garry <john.g.garry@...cle.com>
Subject: [PATCH] md/md-linear: Enable atomic writes

All the infrastructure has already been plumbed to support this for
stacked devices, so just enable the request_queue limits features flag.

A note about chunk sectors for linear arrays:
While it is possible to set a chunk sectors param for building a linear
array, this is for specifying the granularity at which data sectors from
the device are used. It is not the same as a stripe size, like for RAID0. 

As such, it is not appropriate to set chunk_sectors request queue limit to
the same value, as chunk_sectors request limit is a boundary for which
requests cannot straddle.

However, request_queue limit max_hw_sectors is set to chunk sectors, which
almost has the same effect as setting chunk_sectors limit.

Signed-off-by: John Garry <john.g.garry@...cle.com>

diff --git a/drivers/md/md-linear.c b/drivers/md/md-linear.c
index 5d9b081153757..30ac29b990c9b 100644
--- a/drivers/md/md-linear.c
+++ b/drivers/md/md-linear.c
@@ -74,6 +74,7 @@ static int linear_set_limits(struct mddev *mddev)
 	lim.max_hw_sectors = mddev->chunk_sectors;
 	lim.max_write_zeroes_sectors = mddev->chunk_sectors;
 	lim.io_min = mddev->chunk_sectors << 9;
+	lim.features |= BLK_FEAT_ATOMIC_WRITES;
 	err = mddev_stack_rdev_limits(mddev, &lim, MDDEV_STACK_INTEGRITY);
 	if (err)
 		return err;
-- 
2.43.5