Messages in this thread | | | Subject | Re: [PATCH] coredump_filter: add hugepage core dumping | From | Adam Litke <> | Date | Fri, 29 Aug 2008 10:28:33 -0500 |
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On Fri, 2008-08-29 at 08:35 +0900, KOSAKI Motohiro wrote: > Hi > > > > I think VM_RESERVED default dumping bahavior is good, > > > then I'd like to add coredump_filter mask. > > > > > > This patch doesn't change dafault behavior. > > > > This seems very reasonable to me > > (though I've little use for coredumps or hugepages myself). > > Thanks a lot. > > > One caution though: how well does it behave when coredumping a large > > area of hugepages which have not actually been instantiated prior to > > the coredump? We have that funny FOLL_ANON ZERO_PAGE code in > > follow_page() to avoid wasting memory on large uninstantiated anon > > areas, but hugepages won't go that way. If the dump hangs waiting for > > memory to be freed, or OOMkills other processes, that wouldn't be good; > > whereas if hugepage reservations (I've not followed what happens with > > them) or whatever just make it skip when no more, that should be okay. > > I think hugepage reservation pages always exist when hugepage COW happend. > Then, hugepage access never cause try_to_free_pages() nor OOM.
(Mel, since you wrote the private reservation hugetlb code, would you care to verify the following:)
Well, reserved huge pages _almost_ always exist. The notable exception happens when a process creates a MAP_PRIVATE hugetlb mapping and then forks. No guarantees are made to the children for access to that hugetlb mapping. So if such a child were to core dump an unavailable huge page, follow_hugetlb_page() would fail. I think that case is harmless since it looks like elf_coredump() will replace it with a zeroed page?
The part of Hugh's email that does deserve more attention is the part about FOLL_ANON and the ZERO_PAGE. It seems like an awful waste to zero out and instantiate huge pages just for a core dump. I think it would be worth adding a flag to follow_hugetlb_page() so that it can be instructed to not fault in un-instantiated huge pages. This would take some investigating as to whether it is even valid for follow_hugetlb_page() to return the ZERO_PAGE().
-- Adam Litke - (agl at us.ibm.com) IBM Linux Technology Center
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