Re: [PATCH] arch-ppc.h: Add ilog2 implementation for ppc64

[Jens Axboe <axboe@kernel.dk>]

On Tue, Aug 11 2015, Chandan Rajendra wrote:
> On a ppc64 machine, when fio is invoked with the following configuration file
> (generated by fstests' generic/300 test), it loops indefinitely.
> 
> [global]
> directory=/mnt/btrfs-xfstest-scratch
> filesize=536870912
> size=999G
> continue_on_error=write
> ignore_error=,ENOSPC
> error_dump=0
> 
> create_on_open=1
> fallocate=none
> exitall=1
> 
> [direct_aio_raicer]
> ioengine=libaio
> iodepth=128*1
> bs=128k
> direct=1
> numjobs=4
> rw=randwrite
> runtime=100*1
> time_based
> filename=racer
> 
> [falloc_raicer]
> ioengine=falloc
> runtime=100*1
> iodepth=1
> bssplit=128k/80:512k/10:32k/10
> rw=randwrite
> numjobs=1
> filename=racer
> 
> [punch_hole_raicer]
> ioengine=falloc
> runtime=100*1
> bs=4k
> time_based=10
> rw=randtrim
> numjobs=2
> filename=racer
> time_based
> 
> [aio-dio-verifier]
> ioengine=libaio
> iodepth=128*1
> numjobs=1
> verify=crc32c-intel
> verify_fatal=1
> verify_dump=1
> verify_backlog=1024
> verify_async=4
> verifysort=1
> direct=1
> bs=4k
> rw=randwrite
> filename=aio-dio-verifier
> 
> This is because arch_ffz() ends up invoking the 32-bit version of __ilog2()
> which always returns a bit number in the range 0 - 31. This can cause
> "overlap" variable in axmap_set_fn() to never becomes zero.
> 
> To fix this the commit adds a 64-bit version of __ilog2() (obtained from the
> Linux kernel's arch/powerpc/include/asm/bitops.h).

Awesome, thanks! This resolves an issue that someone reported a while
back that only happened on ppc, and this explains it.

-- 
Jens Axboe