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SubjectRe: [PATCH 0/5] x86: another attempt at x86 pagetable unification
Ingo Molnar wrote:
> * Jeremy Fitzhardinge <jeremy@goop.org> wrote:
>
>
>>> found a couple of bugs.
>>>
>>> firstly, 64-bit wasnt so lucky, you broke
>>> iounmap()/change_page_attr()
>>> :-)
>>>
>> Crap. Worked for me. I'll look into it.
>>
>
> well, there's an easy solution for unification patches: the resulting
> object files must have _exactly the same_ content as without the
> unification patches. (Modulo strings as WARN_ON()s referring to
> include-file names.)
>
> If they differ then the unification did something wrong. With your
> patchset and the config i sent, the difference is visible in the image
> size already:
>
> text data bss dec hex filename
> 7763766 967330 5812328 14543424 ddea40 vmlinux.after
> 7763811 967330 5812328 14543469 ddea6d vmlinux.before
>
> also, reducing the size and scope of changes helps as well - because
> that way it can be bisected down to specific changes. Mistakes
> inevitably happen, especially if you do not enforce a rigid
> byte-for-byte correctness along the way. You did 5 rather large patches,
> and it's not testable because your unification steps were too coarse.
>

But byte-for-byte identity isn't (necessarily) possible when actually
unifying. If the same function exists in different forms on 32- and
64-bit, then unifying requires I pick one of them (or perhaps a new
superset) to use in the unified form. That function may generate
different code compared to the one that it replaced...

But you're right, I can do the patches in a more piecemeal form. I'll
see if I can rework them.

J


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