Re: [PATCH v2 2/3] lib/find_bit: create find_first_zero_bit_le()

[Yury Norov <yury.norov@gmail.com>]

On Wed, Aug 24, 2022 at 06:58:10PM +0100, Russell King (Oracle) wrote:
> On Wed, Aug 24, 2022 at 06:37:45AM -0700, Yury Norov wrote:
> > Because there is a codebase that relies on existing types, mostly in
> > filesystem code. And those fs fixes would require 5 or 6 patches.
> 
> Does that mean that are there filesystems that are passing pointers to
> the find_bit functions which are _not_ aligned to an "unsigned long" ?

Not necessarily. For example, I looked at ext2/4 code, and it looks like
they need void *bitmap, because they pass opaque pointer ->b_data from
struct buffer_head:

  struct buffer_head {
        ...
        char *b_data;                   /* pointer to data within the page */
        ...
  }

So, quite probably, the pointer is aligned when it points to a bitmap.
But I have nothing to prove it, except standard anthropic principle
"otherwise people would complain".

In LE case, the find_bit_le() functions are aliased to find_bit(),
which is aligned, and somehow it works.

> If there are, we should _not_ convert 32-bit ARM to use word loads or
> use the generic code; unaligned loads are expensive on older ARM CPUs,
> at least not the code for older ARM CPUs.

I wonder, if there's an arch that throws an exception in case of unaligned
access. IIRC, ARM can do that, but this option is disabled in kernel.
Right?

I have a series that adds a runtime parameters checker for bitmaps.
I'll add a test for alignment and see how it works (not very soon).

Thanks,
Yury