btrfs_get_token_64() alignment problem on ARM (was: Re: DWord alignment on ARMv7)

[David Sterba]

On Fri, Mar 04, 2016 at 09:01:44AM +0100, Marc Kleine-Budde wrote:
> Hello,
> 
> On 03/04/2016 12:54 AM, Will Deacon wrote:
> > Hi Marc,
> > 
> > On Thu, Mar 03, 2016 at 11:27:11PM +0100, Marc Kleine-Budde wrote:
> >> I'm using btrfs on am ARMv7 and it turns out, that the kernel has to
> >> fixup a lot of kernel originated alignment issues.
> >>
> >> See /proc/cpu/alignment (~4h of uptime):
> >>> System: 22304815 (btrfs_get_token_64+0x13c/0x148 [btrfs])
> >>
> >> For example, when compiling the kernel on a btrfs volume the counter
> >> increases by 100...1000 per second.
> >>
> >> The function shown "btrfs_get_token_64()" is defined here:
> >>> http://lxr.free-electrons.com/source/fs/btrfs/struct-funcs.c#L53
> >> ...it already uses get_unaligned_leXX accessors.
> >>
> >> Quoting a comment in arch/arm/mm/alignment.c:
> >>
> >>          * ARMv6 and later CPUs can perform unaligned accesses for
> >>          * most single load and store instructions up to word size.
> >>          * LDM, STM, LDRD and STRD still need to be handled.
> >>
> >> But on a 32bit ARMv7 64bits are not word-sized.
> >>
> >> Is the exception and fixup overhead neglectable? Do we have to introduce
> >> something like HAVE_EFFICIENT_UNALIGNED_64BIT_ACCESS?
> > 
> > Ouch, that trap/emulate is certainly going to have an effect on your
> > performance. I doubt that HAVE_EFFICIENT_UNALIGNED_ACCESS applies to
> > types bigger than the native word size on many architectures, so my
> > hunch is that the btrfs code should be checking BITS_PER_LONG or similar
> > to establish whether or not to break the access up into word accesses.
> 
> I've added the btrfs maintainers on Cc.

Can this be done transparently via the the get_unaligned_le* helpers?
This seems to be too arch-specific to fix it in btrfs.