Re: [PATCH] btrfs: fix balance convert to single on 32-bit host CPUs

[Zygo Blaxell <ce3g8jdj@umail.furryterror.org>]

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On Fri, Sep 13, 2019 at 10:24:08AM +0200, Johannes Thumshirn wrote:
> On 13/09/2019 01:55, Zygo Blaxell wrote:
> > Currently, the command:
> > 
> > 	btrfs balance start -dconvert=single,soft .
> > 
> > on a Raspberry Pi produces the following kernel message:
> > 
> > 	BTRFS error (device mmcblk0p2): balance: invalid convert data profile single
> > 
> > This fails because we use is_power_of_2(unsigned long) to validate
> > the new data profile, the constant for 'single' profile uses bit 48,
> > and there are only 32 bits in a long on ARM.
> > 
> > Fix by open-coding the check using u64 variables.
> > 
> > Tested by completing the original balance command on several Raspberry
> > Pis.
> > 
> > Signed-off-by: Zygo Blaxell <ce3g8jdj@umail.furryterror.org>
> > ---
> >  fs/btrfs/volumes.c | 6 +++++-
> >  1 file changed, 5 insertions(+), 1 deletion(-)
> > 
> > diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
> > index 88a323a453d8..252c6049c6b7 100644
> > --- a/fs/btrfs/volumes.c
> > +++ b/fs/btrfs/volumes.c
> > @@ -3906,7 +3906,11 @@ static int alloc_profile_is_valid(u64 flags, int extended)
> >  		return !extended; /* "0" is valid for usual profiles */
> >  
> >  	/* true if exactly one bit set */
> > -	return is_power_of_2(flags);
> > +	/*
> > +	 * Don't use is_power_of_2(unsigned long) because it won't work
> > +	 * for the single profile (1ULL << 48) on 32-bit CPUs.
> > +	 */
> > +	return flags != 0 && (flags & (flags - 1)) == 0;
> 
> Would
> flags && IS_ALIGNED(flags)
> 
> Work as well? IS_ALIGNED() should be type-save due to the typeof():
> #define IS_ALIGNED(x, a) (((x) & ((typeof(x))(a) - 1)) == 0)
> 
> Or maybe I'm missing something subtle?

Wouldn't that be
	
	flags && IS_ALIGNED(flags, flags)

?

> Thanks,
> 	Johannes
> -- 
> Johannes Thumshirn                            SUSE Labs Filesystems
> jthumshirn@suse.de                                +49 911 74053 689
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