[Bug 281402] zfs: kmsan reports Uninitialized UMA memory from zio_data_buf_alloc

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=281402

            Bug ID: 281402
           Summary: zfs: kmsan reports Uninitialized UMA memory from
                    zio_data_buf_alloc
           Product: Base System
           Version: CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: bugs@FreeBSD.org
          Reporter: asomers@FreeBSD.org

With kmsan enabled, ZFS rapidly reports the use of uninitialized memory
whenever a pool uses both raidz and file-backed vdevs.  For example:

$ truncate -s 1g /tmp/a /tmp/b
$ sudo zpool create testpool raidz /tmp/a /tmp/

That is sufficien to generate warnings like this:

MSan: Uninitialized UMA memory in g_disk_start, offset 0/32768, addr
0xfffffe00454a4000, from zio_data_buf_alloc+0xd8
#0 0xffffffff833e7ffc at kmsan_report_hook+0x15c
#1 0xffffffff82d58ffd at g_disk_start+0x26d
#2 0xffffffff82d6e4f7 at g_io_request+0x13e7
#3 0xffffffff82d6e4f7 at g_io_request+0x13e7
#4 0xffffffff846c75c8 at ufs_strategy+0x418
#5 0xffffffff85427a97 at VOP_STRATEGY_APV+0x107
#6 0xffffffff8368ae75 at bufstrategy+0x195
#7 0xffffffff8368a64a at bufwrite+0xc3a
#8 0xffffffff8369c518 at vfs_bio_awrite+0x848
#9 0xffffffff8466258f at ffs_syncvnode+0x127f
#10 0xffffffff84659f9c at ffs_fsync+0xac
#11 0xffffffff85420a77 at VOP_FSYNC_APV+0x107
#12 0xffffffff877a79c0 at zfs_file_fsync+0x200
#13 0xffffffff86f58ad0 at vdev_file_io_start+0x140
#14 0xffffffff878a8253 at zio_vdev_io_start+0xd53
#15 0xffffffff8787b8c0 at zio_nowait+0x6c0
#16 0xffffffff8787b02c at zio_flush+0x13c
#17 0xffffffff875fa2b3 at vdev_config_sync+0x563
MSan: Uninitialized UMA memory in dmasync, offset 0/32768, addr
0xfffffe00454a4000, from zio_data_buf_alloc+0xd8
#0 0xffffffff833e7ffc at kmsan_report_hook+0x15c
#1 0xffffffff833e7e6b at kmsan_bus_dmamap_sync+0x25b
#2 0xffffffff82759f9f at vtblk_request_execute_cb+0x11df
#3 0xffffffff8336ccb5 at bus_dmamap_load_mem+0x1975
#4 0xffffffff8336b310 at bus_dmamap_load_bio+0x150
#5 0xffffffff82757867 at vtblk_startio+0xb07
#6 0xffffffff8275aae5 at vtblk_strategy+0x175
#7 0xffffffff82d59e08 at g_disk_start+0x1078
#8 0xffffffff82d6e4f7 at g_io_request+0x13e7
#9 0xffffffff82d6e4f7 at g_io_request+0x13e7
#10 0xffffffff846c75c8 at ufs_strategy+0x418
#11 0xffffffff85427a97 at VOP_STRATEGY_APV+0x107
#12 0xffffffff8368ae75 at bufstrategy+0x195
#13 0xffffffff8368a64a at bufwrite+0xc3a
#14 0xffffffff8369c518 at vfs_bio_awrite+0x848
#15 0xffffffff8466258f at ffs_syncvnode+0x127f
#16 0xffffffff84659f9c at ffs_fsync+0xac
#17 0xffffffff85420a77 at VOP_FSYNC_APV+0x107

Unfortunately, zio_data_buf_alloc is called from 37 different places, and I
don't know how to tell which of the callers is responsible for not initializing
its memory.

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