Re: [PATCH 1/2] bitmap: add sanity check function for find_bit()

[Yury Norov]

On Sun, Oct 23, 2022 at 03:19:24PM -0700, Linus Torvalds wrote:
> On Wed, Oct 19, 2022 at 8:24 PM Yury Norov <yury.norov@xxxxxxxxx> wrote:
> >
> > This patch adds runtime check for the pointers to be aligned.
> 
> No. Don't add pointless things like this. It only adds code, with no advantage.

Sure. Patch #1 is mostly for Russell to address his concern about
unaligned bitmaps on ARM32. And it looks like it found nothing.
 
> The bitmap ops all operate on 'unsigned long', and if a bitmap isn't
> aligned, we'll take a fault on the architectures that don't do
> unaligned accesses natively.

ARMv6 may or may not support unaligned access depending on SCTLR.U
bit. This is what Russell was concerned about in the other email.
As far as I understand, linux enables that feature.

ARMv7 deprecates that bit and supports unaligned dereference
unconditionally, with few exceptions like exclusive access.

https://developer.arm.com/documentation/ddi0406/b/Appendices/ARMv6-Differences/Application-level-memory-support/Alignment?lang=en
Thanks,
Yury
 
> And the find-bit functions simply aren't special enough to have this
> kind of random testing, when the *basic* bitmap functions like
> "set_bit()" and friends all do the accesses without any alignment
> checks.
> 
> The fact that filesystem code often uses bitmap functions with a cast
> from 'char *' is immaterial. Those things are already aligned
> (typically they are a whole disk block). They just weren't an array of
> 'unsigned long'.
>
>                 Linus