Re: [PATCHv2] mm: Fix warning in move_normal_pmd()

[Linus Torvalds <torvalds@linux-foundation.org>]

On Wed, Jul 15, 2020 at 1:54 PM Joel Fernandes <joel@joelfernandes.org> wrote:
>
> Regarding the ADDR_AFTER_NEXT checks, shouldn't you check for:
>
>         if (ADDR_AFTER_NEXT(ALIGN(*old_addr + *len, PMD_SIZE), old))
>                 return;

No, there's even a comment to the effect.

Instead, that ADDR_AFTER_NEXT() aligns the next address _down_ to the
PMD boundary.

Because otherwise, what can happen is:

 - you're on an architecture that has a separate address space for users

 - you're the next-to-last VMA in that address space,

 - you're in the last PMD.

And now "ALIGN(*old_addr + *len, PMD_SIZE)" will wrap, and become 0,
and you think it's ok to move the whole PMD, because it's now smaller
than the start address of the next VMA.

It's _not_ ok, because you'd be moving that next-vma data too.

> and for the len calculation, I did not follow what you did, but I think you
> meant something like this? Does the following reduce to what you did? At
> least this is a bit more readable I think:
>
>         *len += (ALIGN(*new_addr + *len, PMD_SIZE) - (*new_addr + *len));

Yes, right you are.

I actually wrote that first (except I added a helper variable for that
"*new_addr + *len" thing), and then I decided it can be simplified.

And simplified it wrong ;)

> Also you did "len +=", it should be "*len +=" in this function.

That's indeed a plain stupid bug ;)

Naresh - don't test that version. The bugs Joel found just make the
math wrong, so it won't work.

The concept was solid, the implementation not so much ;)

                 Linus