brd: Request from fdisk 4k alignment

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.

The call to blk_queue_physical_block_size() is harmless and will
not affect the Kernel behavior in any way. It is only for
communication 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)

Note that with bdev-size > 4M fdisk will try to align on a 1M
boundary (above first-sector will be 2048), in any case.

CC: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Boaz Harrosh <boaz@plexistor.com>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

1 files changed, 9 insertions, 0 deletions

diff --git a/drivers/block/brd.c b/drivers/block/brd.c
index a7463c9595e7..c01b921b1b4a 100644
--- a/drivers/block/brd.c
+++ b/drivers/block/brd.c
@@ -486,10 +486,19 @@ 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
+         * (This is only a problem on very small devices <= 4M,
+         *  otherwise fdisk will align on 1M. Regardless this call
+         *  is harmless)
+         */
+        blk_queue_physical_block_size(brd->brd_queue, PAGE_SIZE);
+
         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;