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diff --git a/open_issues/trust_the_behavior_of_translators.mdwn b/open_issues/trust_the_behavior_of_translators.mdwn new file mode 100644 index 00000000..454c638b --- /dev/null +++ b/open_issues/trust_the_behavior_of_translators.mdwn @@ -0,0 +1,181 @@ +[[!meta copyright="Copyright © 2012 Free Software Foundation, Inc."]] + +[[!meta license="""[[!toggle id="license" text="GFDL 1.2+"]][[!toggleable +id="license" text="Permission is granted to copy, distribute and/or modify this +document under the terms of the GNU Free Documentation License, Version 1.2 or +any later version published by the Free Software Foundation; with no Invariant +Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license +is included in the section entitled [[GNU Free Documentation +License|/fdl]]."]]"""]] + +[[!tag open_issue_hurd]] + +Apart from the issue of [[translators_set_up_by_untrusted_users]], here is +another problem described. + + +# IRC, freenode, #hurd, 2012-02-17 + +(Preceded by the [[memory_object_model_vs_block-level_cache]] discussion.) + + <slpz> what should do Mach with a translator that doesn't clean pages in a + reasonable amount of time? + <slpz> (I'm talking about pages flushed to a pager by the pageout daemon) + <braunr> slpz: i don't know what it should do, but currently, it uses the + default pager + +[[default_pager]]. + + <slpz> braunr: I know, but I was thinking about an alternative, for the + case in which a translator in not behaving properly + <slpz> perhaps freeing the page, removing the map, and letting it die in a + segmentation fault could be an option + <braunr> slpz: what if the translator can't do it properly because of + system resource exhaustion ? + <braunr> (i.e. it doesn't have enough cpu time) + <slpz> braunr: that's the biggest question + <slpz> let's suppose that Mach selects a page, sends it to the pager for + cleaning it up, reinjects the page into the active queue, and later it + founds the page again and it's still dirty + <slpz> but it needs to free some pages because memory it's really, really + scarce + <slpz> Linux just sits there waiting for I/O completion for that page + (trusts its own drivers) + <slpz> but we could be dealing with rogue translator... + <braunr> yes + <braunr> we may need some sort of "authentication" mechanism for pagers + <braunr> so that "system pagers" are trusted and others not + <braunr> using something like the device master port but for pagers + <braunr> a special port passed to trusted pagers only + <slpz> hum... that could be used to workaround the untrusted translator + crossing problem while walking a directory tree + +[[translators_set_up_by_untrusted_users]]. + + <slpz> but I think differentiating between trusted and untrusted + translators was rejected for philosophical reasons + <slpz> (but I'm not sure) + <mcsim> slpz: probably there should be something like oom killer? + <mcsim> braunr: even if translator is trusted it could have a bug which + make it ask more and more memory, so system have something to do with + it. Also, this way TCB is increased, so providing port for trusted + translators may hurt security. + <mcsim> I've read that Genode has "guarded allocators" which help resource + accounting by limiting of memory that could be used. Probably something + like this could be used in Hurd to limit translators. + <antrik> I don't remember how Viengoos deals with this :-( + +[[microkernel/Viengoos]]. + + <braunr> mcsim: the main feature lacking in mach is resource accounting :p + +[[resource_management_problems]]. + + <slpz> mcsim: yes, I think there should be a Hurdish oom killer, paying + special attention to external pagers + +[[microkernel/mach/external_pager_mechanism]]. + + <braunr> the oom killer selects untrusted processes by definition (since + pagers are in kernel) + <mcsim> slpz: and what is better: oom killer or resource accounting? + <mcsim> Under resource accounting I mean mechanism when process can't get + more resources than it is allowed. + <braunr> resource accounting of course + <braunr> but it's not just about that + <braunr> really, how does the kernel deal when a pager refuses to honor a + paging request ? + <braunr> whether it is buggy or malicious + <braunr> is it really possible to keep all pagers out of the TCB ? + <antrik> mcsim: we definitely want proper resource accounting in the long + run. the question is how to deal with the situation that resources are + reallocated to other tasks, so some pages have to be freed + <antrik> I really don't remember how Neal proposed to deal with this + <slpz> mcsim: Better: resource accounting (in which resources are accounted + to the user tasks which are requesting them, as in the Viengoos + model). Good enough an realistic: oom killer + <antrik> I'm not sure an OOM killer for non-system pagers is terribly + helpful. in typical use, the vast majority of paging is done by trusted + pagers... + <antrik> and without proper client resource accounting, there are enough + other ways a rogue/broken process can eat system resources -- I'm not + convinced that untrusted pagers have a major impact on the overall + situation + <mcsim> If pager can't free resources because of lack, for example, of cpu + time it's priority could be increased to give it second chance to free + the page. But if it doesn't manage to free resources it could be killed. + <antrik> I think the current approach with default pager taking over is + good enough for dealing with untrusted pagers. the real problem are even + trusted pager frequently failing to deal with the requests + <braunr> i agree with antrik + <braunr> and i'm opposed to an oom killer + <braunr> it's really not a proper fix for our problems + <braunr> mcsim: what if needs 3 attempts before succeeding ? + <braunr> +it + <braunr> and increasing priority without a good reason (e.g. no priority + inversion) leads to unfairness + <braunr> we don't want to deal with tricky problems like malicious pagers + using that to starve other tasks + <mcsim> braunr: this is just temporary decision (for example for half of + second of user time), to increase probability that task was killed not + because of it lacked resources. + <braunr> mcsim: tunables should only affect the efficiency of an operation, + not its success + + +## IRC, freenode, #hurd, 2012-02-19 + + <antrik> neal: the specific question is how to ensure processes free memory + fast enough when their allocation becomes lower due to resource pressure + <neal> antrik: you can't really. + <neal> antrik: the memory manager can act on the application's behalf if + the application marks pages as discardable or pagable. + <neal> antrik: if the memory manager makes an upcall to the application to + free some memory and it doesn't, you have to penalize it. + <neal> antrik: You shouldn't the process like exokernel + <neal> antrik: It's the developers fault, not the user's + <neal> antrik: What you need are controls that ensure that the user stays + in control + <neal> ...shouldn't *kill* the process... + <antrik> neal: well, how can I penalize a process that eats to much + physical memory? + <neal> in the future, you don't give it as much slack memory + <antrik> marking as pagable means a system pager will push them to the swap + partition? + <antrik> ah, OK + <neal> yes + <neal> and you page it more aggressively, i.e., you don't give it a chance + to free memory + <neal> The situation is: + <neal> you have memory pressure + <neal> you choose a victim process and ask it to free memory + <neal> now, you need to wait + <neal> if you wait and it doesn't free memory, you give it bad karma + <neal> if you wait and it frees memory, you're good + <neal> but during that window, a bad process can delay recovery + <neal> so, in the future, we give bad processes less time + <neal> but, we still send a message asking it to free memory + <neal> we just hope it was a bug + <antrik> so the major difference to the approach we have in Mach is that + instead of just redeclaring some pages as anonymous memory that will be + paged to swap by the default pager eventually if the pager in question + fails to handle them properly, we wait some time for the process to free + (any) memory, and only then start paging out some of it's pages to swap + <neal> there's also discardable memory + <antrik> hm... there is some notion of "precious" pages in Mach... I wonder + whether that is also used to decide about discarding pages instead of + pushing them to swap? + <neal> antrik: A precious page is ro data that shouldn't be dropped + <antrik> ah + <antrik> but I guess that means non-precious RO data (such as a cache) can + be dropped without asking the pager, right? + <neal> yes + <antrik> I wonder whether such a karma system can be introduced in Mach as + well to deal with problematic pagers + + +## IRC, freenode, #hurd, 2012-02-21 + + <neal> antrik: One of the main differences between Mach and Viengoos is + that in Mach servers are responsible for managing memory whereas in + Viengoos applications are primarily responsible for managing memory. |