From b0245d6478ea5906e3d7a542244d5c015fd47bc7 Mon Sep 17 00:00:00 2001 From: Eric Blake Date: Sat, 30 Mar 2019 20:36:36 -0500 Subject: [PATCH] nbd/server: Advertise actual minimum block size Both NBD_CMD_BLOCK_STATUS and structured NBD_CMD_READ will split their reply according to bdrv_block_status() boundaries. If the block device has a request_alignment smaller than 512, but we advertise a block alignment of 512 to the client, then this can result in the server reply violating client expectations by reporting a smaller region of the export than what the client is permitted to address (although this is less of an issue for qemu 4.0 clients, given recent client patches to overlook our non-compliance at EOF). Since it's always better to be strict in what we send, it is worth advertising the actual minimum block limit rather than blindly rounding it up to 512. Note that this patch is not foolproof - it is still possible to provoke non-compliant server behavior using: $ qemu-nbd --image-opts driver=blkdebug,align=512,image.driver=file,image.filename=/path/to/non-aligned-file That is arguably a bug in the blkdebug driver (it should never pass back block status smaller than its alignment, even if it has to make multiple bdrv_get_status calls and determine the least-common-denominator status among the group to return). It may also be possible to observe issues with a backing layer with smaller alignment than the active layer, although so far I have been unable to write a reliable iotest for that scenario (but again, an issue like that could be argued to be a bug in the block layer, or something where we need a flag to bdrv_block_status() to state whether the result must be aligned to the current layer's limits or can be subdivided for accuracy when chasing backing files). Anyways, as blkdebug is not normally used, and as this patch makes our server more interoperable with qemu 3.1 clients, it is worth applying now, even while we still work on a larger patch series for the 4.1 timeframe to have byte-accurate file lengths. Note that the iotests output changes - for 223 and 233, we can see the server's better granularity advertisement; and for 241, the three test cases have the following effects: - natural alignment: the server's smaller alignment is now advertised, and the hole reported at EOF is now the right result; we've gotten rid of the server's non-compliance - forced server alignment: the server still advertises 512 bytes, but still sends a mid-sector hole. This is still a server compliance bug, which needs to be fixed in the block layer in a later patch; output does not change because the client is already being tolerant of the non-compliance - forced client alignment: the server's smaller alignment means that the client now sees the server's status change mid-sector without any protocol violations, but the fact that the map shows an unaligned mid-sector hole is evidence of the block layer problems with aligned block status, to be fixed in a later patch Signed-off-by: Eric Blake Message-Id: <20190329042750.14704-7-eblake@redhat.com> Reviewed-by: Vladimir Sementsov-Ogievskiy [eblake: rebase to enhanced iotest 241 coverage] --- nbd/server.c | 13 ++++++++----- tests/qemu-iotests/223.out | 4 ++-- tests/qemu-iotests/233.out | 2 +- tests/qemu-iotests/241.out | 10 ++++++---- 4 files changed, 17 insertions(+), 12 deletions(-) diff --git a/nbd/server.c b/nbd/server.c index fd013a2817..218a2aa5e6 100644 --- a/nbd/server.c +++ b/nbd/server.c @@ -607,13 +607,16 @@ static int nbd_negotiate_handle_info(NBDClient *client, uint16_t myflags, /* Send NBD_INFO_BLOCK_SIZE always, but tweak the minimum size * according to whether the client requested it, and according to * whether this is OPT_INFO or OPT_GO. */ - /* minimum - 1 for back-compat, or 512 if client is new enough. - * TODO: consult blk_bs(blk)->bl.request_alignment? */ - sizes[0] = - (client->opt == NBD_OPT_INFO || blocksize) ? BDRV_SECTOR_SIZE : 1; + /* minimum - 1 for back-compat, or actual if client will obey it. */ + if (client->opt == NBD_OPT_INFO || blocksize) { + sizes[0] = blk_get_request_alignment(exp->blk); + } else { + sizes[0] = 1; + } + assert(sizes[0] <= NBD_MAX_BUFFER_SIZE); /* preferred - Hard-code to 4096 for now. * TODO: is blk_bs(blk)->bl.opt_transfer appropriate? */ - sizes[1] = 4096; + sizes[1] = MAX(4096, sizes[0]); /* maximum - At most 32M, but smaller as appropriate. */ sizes[2] = MIN(blk_get_max_transfer(exp->blk), NBD_MAX_BUFFER_SIZE); trace_nbd_negotiate_handle_info_block_size(sizes[0], sizes[1], sizes[2]); diff --git a/tests/qemu-iotests/223.out b/tests/qemu-iotests/223.out index 7828a44739..d5201b2356 100644 --- a/tests/qemu-iotests/223.out +++ b/tests/qemu-iotests/223.out @@ -41,7 +41,7 @@ exports available: 2 export: 'n' size: 4194304 flags: 0x4ef ( readonly flush fua trim zeroes df cache ) - min block: 512 + min block: 1 opt block: 4096 max block: 33554432 available meta contexts: 2 @@ -50,7 +50,7 @@ exports available: 2 export: 'n2' size: 4194304 flags: 0x4ed ( flush fua trim zeroes df cache ) - min block: 512 + min block: 1 opt block: 4096 max block: 33554432 available meta contexts: 2 diff --git a/tests/qemu-iotests/233.out b/tests/qemu-iotests/233.out index 5acbc13b54..9511b6ea65 100644 --- a/tests/qemu-iotests/233.out +++ b/tests/qemu-iotests/233.out @@ -38,7 +38,7 @@ exports available: 1 export: '' size: 67108864 flags: 0x4ed ( flush fua trim zeroes df cache ) - min block: 512 + min block: 1 opt block: 4096 max block: 33554432 available meta contexts: 1 diff --git a/tests/qemu-iotests/241.out b/tests/qemu-iotests/241.out index f22eabbf32..f481074a02 100644 --- a/tests/qemu-iotests/241.out +++ b/tests/qemu-iotests/241.out @@ -3,8 +3,9 @@ QA output created by 241 === Exporting unaligned raw image, natural alignment === size: 1024 - min block: 512 -[{ "start": 0, "length": 1024, "depth": 0, "zero": false, "data": true, "offset": OFFSET}] + min block: 1 +[{ "start": 0, "length": 1000, "depth": 0, "zero": false, "data": true, "offset": OFFSET}, +{ "start": 1000, "length": 24, "depth": 0, "zero": true, "data": true, "offset": OFFSET}] 1 KiB (0x400) bytes allocated at offset 0 bytes (0x0) === Exporting unaligned raw image, forced server sector alignment === @@ -20,7 +21,8 @@ WARNING: Image format was not specified for '/home/eblake/qemu/tests/qemu-iotest === Exporting unaligned raw image, forced client sector alignment === size: 1024 - min block: 512 -[{ "start": 0, "length": 1024, "depth": 0, "zero": false, "data": true, "offset": OFFSET}] + min block: 1 +[{ "start": 0, "length": 1000, "depth": 0, "zero": false, "data": true, "offset": OFFSET}, +{ "start": 1000, "length": 24, "depth": 0, "zero": true, "data": true, "offset": OFFSET}] 1 KiB (0x400) bytes allocated at offset 0 bytes (0x0) *** done -- 2.39.2