Messages in this thread | | | Subject | Re: [RFC PATCH 0/5] madvise MADV_DOEXEC | From | David Hildenbrand <> | Date | Mon, 16 Aug 2021 14:42:24 +0200 |
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On 16.08.21 14:20, David Hildenbrand wrote: > On 16.08.21 14:07, Matthew Wilcox wrote: >> On Mon, Aug 16, 2021 at 10:02:22AM +0200, David Hildenbrand wrote: >>>> Mappings within this address range behave as if they were shared >>>> between threads, so a write to a MAP_PRIVATE mapping will create a >>>> page which is shared between all the sharers. The first process that >>>> declares an address range mshare'd can continue to map objects in the >>>> shared area. All other processes that want mshare'd access to this >>>> memory area can do so by calling mshare(). After this call, the >>>> address range given by mshare becomes a shared range in its address >>>> space. Anonymous mappings will be shared and not COWed. >>> >>> Did I understand correctly that you want to share actual page tables between >>> processes and consequently different MMs? That sounds like a very bad idea. >> >> That is the entire point. Consider a machine with 10,000 instances >> of an application running (process model, not thread model). If each >> application wants to map 1TB of RAM using 2MB pages, that's 4MB of page >> tables per process or 40GB of RAM for the whole machine. > > What speaks against 1 GB pages then? > >> >> There's a reason hugetlbfs was enhanced to allow this page table sharing. >> I'm not a fan of the implementation as it gets some locks upside down, >> so this is an attempt to generalise the concept beyond hugetlbfs. > > Who do we account the page tables to? What are MADV_DONTNEED semantics? > Who cleans up the page tables? What happens during munmap? How does the > rmap even work? How to we actually synchronize page table walkers? > > See how hugetlbfs just doesn't raise these problems because we are > sharing pages and not page tables?
I found what you were referring to: CONFIG_ARCH_WANT_HUGE_PMD_SHARE
I was not aware that we have such a monstrosity in the kernel.
-- Thanks,
David / dhildenb
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