From fd8d372dd36e839568a718684914d9960d8b1ebd Mon Sep 17 00:00:00 2001 From: Eric Blake Date: Wed, 14 Feb 2018 21:29:05 -0600 Subject: [PATCH] nbd: Honor server's advertised minimum block size Commit 79ba8c98 (v2.7) changed the setting of request_alignment to occur only during bdrv_refresh_limits(), rather than at at bdrv_open() time; but at the time, NBD was unaffected, because it still used sector-based callbacks, so the block layer defaulted NBD to use 512 request_alignment. Later, commit 70c4fb26 (also v2.7) changed NBD to use byte-based callbacks, without setting request_alignment. This resulted in NBD using request_alignment of 1, which works great when the server supports it (as is the case for qemu-nbd), but falls apart miserably if the server requires alignment (but only if qemu actually sends a sub-sector request; qemu-io can do it, but most qemu operations still perform on sectors or larger). Even later, the NBD protocol was updated to document that clients should learn the server's minimum alignment during NBD_OPT_GO; and recommended that clients should assume a minimum size of 512 unless the server understands NBD_OPT_GO and replied with a smaller size. Commit 081dd1fe (v2.10) attempted to do that, by assigning request_alignment to whatever was learned from the server; but it has two flaws: the assignment is done during bdrv_open() so it gets unconditionally wiped out back to 1 during any later bdrv_refresh_limits(); and the code is not using a default of 512 when the server did not report a minimum size. Fix these issues by moving the assignment to request_alignment to the right function, and by using a sane default when the server does not advertise a minimum size. CC: qemu-stable@nongnu.org Signed-off-by: Eric Blake Message-Id: <20180215032905.27146-1-eblake@redhat.com> Reviewed-by: Vladimir Sementsov-Ogievskiy --- block/nbd-client.c | 3 --- block/nbd.c | 2 ++ 2 files changed, 2 insertions(+), 3 deletions(-) diff --git a/block/nbd-client.c b/block/nbd-client.c index 9206652e45..7b68499b76 100644 --- a/block/nbd-client.c +++ b/block/nbd-client.c @@ -846,9 +846,6 @@ int nbd_client_init(BlockDriverState *bs, if (client->info.flags & NBD_FLAG_SEND_WRITE_ZEROES) { bs->supported_zero_flags |= BDRV_REQ_MAY_UNMAP; } - if (client->info.min_block > bs->bl.request_alignment) { - bs->bl.request_alignment = client->info.min_block; - } qemu_co_mutex_init(&client->send_mutex); qemu_co_queue_init(&client->free_sema); diff --git a/block/nbd.c b/block/nbd.c index ef81a9f53b..69b5fd5e8f 100644 --- a/block/nbd.c +++ b/block/nbd.c @@ -474,8 +474,10 @@ static int nbd_co_flush(BlockDriverState *bs) static void nbd_refresh_limits(BlockDriverState *bs, Error **errp) { NBDClientSession *s = nbd_get_client_session(bs); + uint32_t min = s->info.min_block; uint32_t max = MIN_NON_ZERO(NBD_MAX_BUFFER_SIZE, s->info.max_block); + bs->bl.request_alignment = min ? min : BDRV_SECTOR_SIZE; bs->bl.max_pdiscard = max; bs->bl.max_pwrite_zeroes = max; bs->bl.max_transfer = max;