linux-kernel - Re: [GIT PULL] Bitmap patches for v6.0-rc1

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Date:   Thu, 11 Aug 2022 23:15:32 -0700
From:   Yury Norov <yury.norov@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [GIT PULL] Bitmap patches for v6.0-rc1

On Sun, Aug 07, 2022 at 05:12:33PM -0700, Linus Torvalds wrote:
> On Thu, Aug 4, 2022 at 7:29 PM Yury Norov <yury.norov@...il.com> wrote:
> >
> >   https://github.com/norov/linux.git/ tags/bitmap-6.0-rc1
> 
> Grr.
> 
> So I delayed this because I was anticipating pain, but it was worse
> than expected.
> 
> I think I've sorted things out, but part of sorting things out was to
> just basically undo this:
> 
> >       lib/bitmap: change type of bitmap_weight to unsigned long
> 
> which just was pointless and the explanation for it didn't match what it did.
> 
> The explanation was "unsigned makes sense". Ok.
> 
> But what it *did* was not to make the return value unsigned, but to
> also expand it from "int" to "long".
> 
> That did *not* make sense, and caused pointless changes to printf strings.
> 
> And - surprise surprise - new users had come in since, so there would
> have been even *more* pointless changes to printf strings just to
> resolve those.

That's true. I checked the generated code for arm64 and x86, and long
is nothing better than int. I was sure that there was a difference, but
it's obviously a false memory.

I wish I could rebase everything on top of origin/master before
sending a pull request. It would help me make sure that everything
will be OK on your side, but it doesn't work for you for other reasons.
 
> So I basically undid almost all of it. I did leave the "unsigned"
> part, but removed the "long" part, so at least the printf strings
> noise didn't happen.
> 
> That still caused the type clash in the 'min()' in the
> mellanox/mlx4/fw.c file, but at least I couldn't find any new cases of
> that pattern, so it seemed acceptable.
> 
> Can bitmap sizes in theory be "unsigned long" bits and thus have
> "weight" not fit in "int"? Yes. But it's not a realistic concern, I
> feel, and I really really didn't want to deal with this pointless
> churn.

Many bitmap functions use int type to store the length, so currently
bitmaps can't be larger than INT_MAX, even in theory.
 
> And honestly, this was enough churn that I seriously just considered
> throwing the whole pull request away.  And I feel like I will have to
> do a few other architecture tests too before I actually push this out,
> so I may end up still doing that if that shows something else wrong.
> 
> So for any future pull requests, please
> 
>  (a) make them more targeted
> 
>  (b) avoids things like that return value change that clearly was just
> churn and affected random files in the tree
> 
> because right now I'm left with a "never again" feeling about this all.

I understand that. I'm not happy as well. I'll be more careful with
pull requests. Sorry again.

Thanks,
Yury