Re: [PATCH -mm] vmscan: make mapped executable pages the first class citizen

Elladan
2009-05-09T04:06:33Z

On Fri, May 08, 2009 at 12:04:27PM -0400, Rik van Riel wrote:
> Elladan wrote:
>
>>> Nobody (except you) is proposing that we completely disable
>>> the eviction of executable pages. I believe that your idea
>>> could easily lead to a denial of service attack, with a user
>>> creating a very large executable file and mmaping it.
>>>
>>> Giving executable pages some priority over other file cache
>>> pages is nowhere near as dangerous wrt. unexpected side effects
>>> and should work just as well.
>>
>> I don't think this sort of DOS is relevant for a single user or trusted user
>> system.
>
> Which not all systems are, meaning that the mechanism
> Christoph proposes can never be enabled by default and
> would have to be tweaked by the user.
>
> I prefer code that should work just as well 99% of the
> time, but can be enabled by default for everybody.
> That way people automatically get the benefit.

I read Christopher's proposal as essentially, "have a desktop switch which
won't evict executable pages unless they're using more than some huge
percentage of RAM" (presumably, he wants anonymous pages to get special
treatment too) -- this would essentially be similar to mlocking all your
executables, only with a safety net if you go above x% and without affecting
non-executable file maps.

Given that, the DOS possibility you proposed seemed to just be one where a user
could push a lot of unprotected pages out quickly and make the system run slow.

I don't see how that's any different than just asking malloc() for a lot of ram
and then touching it a lot to make it appear very hot to the VM. Any user can
trivially do that already, and some apps (eg. a jvm) happily do that for you.
The pathology is the same, and if anything an executable mmap is harder.

-E