lkml.org 
[lkml]   [2011]   [Mar]   [14]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
Date
SubjectRe: [PATCH] mm: PageBuddy and mapcount underflows robustness
On Mon, Mar 14, 2011 at 9:59 AM, Andrea Arcangeli <aarcange@redhat.com> wrote:
>
> +#define PAGE_BUDDY_MAPCOUNT_VALUE (-1024*1024)

I realize that this is a nitpick, but from a code generation
standpoint, large random constants like these are just nasty.

I would suggest aiming for constants that are easy to generate and/or
fit better in the code stream. In many encoding schemes (eg x86), -128
is much easier to generate, since it fits in a signed byte and allows
small instructions etc. And in most RISC encodings, 8- or 16-bit
constants can be encoded much more easily than something like your
current one, and bigger ones often end up resulting in a load from
memory or at least several immediate-building instructions.

> -       __ClearPageBuddy(page);
> +       if (PageBuddy(page)) /* __ClearPageBuddy VM_BUG_ON(!PageBuddy(page)) */
> +               __ClearPageBuddy(page);

Also, this is just disgusting. It adds no safety here to have that
VM_BUG_ON(), so it's just unnecessary code generation to do this.
Also, we don't even WANT to do that stupid "__ClearPageBuddy()" in the
first place! What those two code-sites actually want are just a simple

reset_page_mapcount(page);

which does the right thing in _general_, and not just for the buddy
case - we want to reset the mapcount for other reasons than just
pagebuddy (ie the underflow/overflow case).

And it avoids the VM_BUG_ON() too, making the crazy conditionals be not needed.

No?

Linus
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2011-03-14 18:39    [W:0.073 / U:5.276 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site