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drbd: always set BLK_FEAT_STABLE_WRITES

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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: 1a02f3a73f ("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>
This commit is contained in:
Christoph Böhmwalder 2026-02-05 18:39:29 +01:00 • committed by Jens Axboe
parent 192c015940
commit 2ebc8d600f
2 changed files with 19 additions and 4 deletions
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3
drivers/block/drbd/drbd_main.c
View file

@ -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);

20
drivers/block/drbd/drbd_nl.c
View file

@ -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