Re: objrmap-core-1 (rmap removal for file mappings to avoid 4:4 in <=16G machines)

[Ingo Molnar <mingo@elte.hu>]

* Andrea Arcangeli <andrea@suse.de> wrote:

> the quality of such objrmap patch is still better than rmap. The DoS
> thing is doable with vmtruncate too in any kernel out there.

objrmap for now has a serious problem: test-mmap3.c locked up my box (i
couldnt switch text consoles for 30 minutes when i turned the box off).

I'm sure you'll fix it and i'm looking forward seeing it.  However, i'd
like to see the full fix instead of a promise to have this fixed
sometime the future.  There are valid application workloads that trigger
_worse_ vma patterns than test-mmap3.c does (UML being one such thing,
Oracle with indirect buffer-cache another - i'm sure there are other
apps too.).  Calling these applications 'exploits' doesnt help in
getting this thing fixed.  There's no problem with keeping this patchset
separate until it's regression-free.

> merging objrmap is the first step. Any other effort happens on top of
> it.

i'd like to see that effort combined with this code, and the full
picture.  Since this 'DoS property' is created by the current concept of
the patch, it's not a 'bug' that is easily fixed so we must not (and
cannot) sign up for it blindly, without seeing the full impact.  But
yes, it might be fixable.  Anyway - the 2.6 kernel is a stable tree and
i'm sure you know that avoiding regression is more important than
anything else.

	Ingo