[PATCH RFC] nvmet: fix tmpfs-based backstores support

[Maurizio Lombardi]

On Thu, Nov 18, 2021 at 01:59:29AM +0000, Chaitanya Kulkarni wrote:
> We will need blktests for this.
> 
> Also, can you share performance numbers with this patch and
> the errors that you are getting without this patch ?

Ok, I will try to run blktests and provide some performance numbers.

Without this patch, writes get lost:

[root at kvm-02-guest03 ~]# nvme connect -t tcp -a 10.37.153.75 -n test-nqn -s 4080
[root at kvm-02-guest03 ~]# dd if=/dev/urandom of=/dev/nvme0n1 bs=1M count=1
1+0 records in
1+0 records out
1048576 bytes (1.0 MB, 1.0 MiB) copied, 0.0604551 s, 17.3 MB/s
[root at kvm-02-guest03 ~]# hexdump -C /dev/nvme0n1 
00000000  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
06400000

dmesg output:

[81489.687548] blk_update_request: critical target error, dev nvme0c0n1, sector 0 op 0x1:(WRITE) flags 0x2000800 phys_seg 256 prio class 0
[81489.691139] buffer_io_error: 246 callbacks suppressed
[81489.691141] Buffer I/O error on dev nvme0n1, logical block 0, lost async page write
[81489.695013] Buffer I/O error on dev nvme0n1, logical block 1, lost async page write
[81489.697183] Buffer I/O error on dev nvme0n1, logical block 2, lost async page write
[81489.699239] Buffer I/O error on dev nvme0n1, logical block 3, lost async page write
[81489.701297] Buffer I/O error on dev nvme0n1, logical block 4, lost async page write
[81489.703374] Buffer I/O error on dev nvme0n1, logical block 5, lost async page write
[81489.705459] Buffer I/O error on dev nvme0n1, logical block 6, lost async page write
[81489.707511] Buffer I/O error on dev nvme0n1, logical block 7, lost async page write
[81489.709438] Buffer I/O error on dev nvme0n1, logical block 8, lost async page write
[81489.711364] Buffer I/O error on dev nvme0n1, logical block 9, lost async page write

Maurizio