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Subject: [private] Re: [RFC PATCH 0/5] madvise MADV_DOEXEC
To:     Matthew Wilcox <willy@infradead.org>
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From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
Message-ID: <86ebdfe4-22ee-9a9a-9d06-8305290d9bc9@redhat.com>
Date:   Mon, 16 Aug 2021 14:27:41 +0200
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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.

Note that I am working on asynchronous reclaim of page tables, whereby I 
would even reclaim !anonymous page tables under memory pressure.

Assuming your processes don't touch all memory all the time of course 
... of course, it's a research project and will still require quite some 
work because devil is in the detail (locking).

-- 
Thanks,

David / dhildenb