linux-kernel - [PATCH 4/5] brd: Request from fdisk 4k alignment

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Date:	Wed, 05 Nov 2014 16:08:18 +0200
From:	Boaz Harrosh <boaz@...xistor.com>
To:	Jens Axboe <axboe@...com>, Matthew Wilcox <willy@...ux.intel.com>,
	Dmitry Monakhov <dmonakhov@...nvz.org>
CC:	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: [PATCH 4/5] brd: Request from fdisk 4k alignment

From: Boaz Harrosh <boaz@...xistor.com>

Because of the direct_access() API which returns a PFN. partitions
better start on 4K boundary, else offset ZERO of a partition will
not be aligned and blk_direct_access() will fail the call.

By setting blk_queue_physical_block_size(PAGE_SIZE) we can communicate
this to fdisk and friends.

Note that blk_queue_physical_block_size() also trashes io_min, but
we can leave this one to be 512. io_min as opposed to physical_block_size
will actually change Kernel behavior, but physical_block_size is not
used by Kernel except being exported to user-mode.

before this patch running fdisk on a default size brd of 4M
the first sector offered is 34 (BAD), but after this patch it
will be 40, ie 8 sectors aligned. Also when entering some random
partition sizes the next partition-start sector is offered 8 sectors
aligned after this patch. (Please note that with fdisk the user
can still enter bad values, only the offered default values will
be correct)

Signed-off-by: Boaz Harrosh <boaz@...xistor.com>
---
 drivers/block/brd.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/block/brd.c b/drivers/block/brd.c
index a7463c959..0026998 100644
--- a/drivers/block/brd.c
+++ b/drivers/block/brd.c
@@ -486,10 +486,17 @@ static struct brd_device *brd_alloc(int i)
 	brd->brd_queue = blk_alloc_queue(GFP_KERNEL);
 	if (!brd->brd_queue)
 		goto out_free_dev;
+
 	blk_queue_make_request(brd->brd_queue, brd_make_request);
 	blk_queue_max_hw_sectors(brd->brd_queue, 1024);
 	blk_queue_bounce_limit(brd->brd_queue, BLK_BOUNCE_ANY);

+	/* This is so fdisk will align partitions on 4k, because of
+	 * direct_access API needing 4k alignment, returning a PFN
+	 */
+	blk_queue_physical_block_size(brd->brd_queue, PAGE_SIZE);
+	brd->brd_queue->limits.io_min = 512; /* Don't use the accessor */
+
 	brd->brd_queue->limits.discard_granularity = PAGE_SIZE;
 	brd->brd_queue->limits.max_discard_sectors = UINT_MAX;
 	brd->brd_queue->limits.discard_zeroes_data = 1;
-- 
1.9.3

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