Re: [RFC PATCH v2 0/5] Add buffered write-through support to iomap & xfs

[Pankaj Raghav (Samsung)]

> 
> 
> * Testing Notes (UPDATED) *
> 
> - I've added support for RWF_WRITETHROUGH to fsx and fsstress in
>   xfstests and these patches survive fsx with integrity verification as
>   well as fsstress parallel stressing.
> - -g quick with blocks size == page size and blocksize < pagesize shows
>   no new regressions.

I am hitting a very strange error in generic/127. I deconstructed it in
to two commands that can reproduce the issue:

```
$ dd if=/dev/zero of=/media/test/fsx_lite_nommap bs=262144 count=1

$ /root/home/xfstests/ltp/fsx -l 262144 -o 65536 -S 191110531 -N 100000 -L -R -W /media/test/fsx_lite_nommap
mapped writes DISABLED
Seed set to 191110531
main: filesystem does not support exchange range, disabling!
main: atomic writes need O_DIRECT (-Z), disabling!
short read: 0x0 bytes instead of 0x6c7f
LOG DUMP (1 total operations):
1(  1 mod 256): READ     0x39381 thru 0x3ffff   (0x6c7f bytes)
Log of operations saved to "/media/test/fsx_lite_nommap.fsxops"; replay with --replay-ops
fsx: save_buffer: .fsxgood file too short... will save 0x0 bytes instead of 0x40000
: Operation not supported
Correct content saved for comparison
(maybe hexdump "/media/test/fsx_lite_nommap" vs "/media/test/fsx_lite_nommap.fsxgood")
```

This is on a bs == ps (4k block size) xfs filesystem.
```
$ xfs_info /media/test/
meta-data=/dev/nvme1n1           isize=512    agcount=4, agsize=2097152 blks
         =                       sectsz=4096  attr=2, projid32bit=1
         =                       crc=1        finobt=1, sparse=1, rmapbt=0
         =                       reflink=1    bigtime=1 inobtcount=1 nrext64=0
data     =                       bsize=4096   blocks=8388608, imaxpct=25
         =                       sunit=0      swidth=0 blks
naming   =version 2              bsize=4096   ascii-ci=0, ftype=1
log      =internal log           bsize=4096   blocks=16384, version=2
         =                       sectsz=4096  sunit=1 blks, lazy-count=1
realtime =none                   extsz=4096   blocks=0, rtextents=0
```

The same test passes if I pass `-G` to fsx parameters(disabling writethrough).

The error is coming when we do a read operation which feels very
strange. Could you run this in your setup and see if you can reproduce
them? I still do not know where the issue is coming from or it is
because of my test setup.

--
Pankaj