Messages in this thread | | | Date | Fri, 8 May 2009 21:04:18 -0700 | From | Elladan <> | Subject | Re: [PATCH -mm] vmscan: make mapped executable pages the first class citizen |
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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
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