linux-kernel - Re: [private] Re: [RFC PATCH 0/5] madvise MADV

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Date:   Mon, 16 Aug 2021 14:30:37 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Khalid Aziz <khalid.aziz@...cle.com>,
        "Longpeng (Mike, Cloud Infrastructure Service Product Dept.)" 
        <longpeng2@...wei.com>, Steven Sistare <steven.sistare@...cle.com>,
        Anthony Yznaga <anthony.yznaga@...cle.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "Gonglei (Arei)" <arei.gonglei@...wei.com>
Subject: Re: [private] Re: [RFC PATCH 0/5] madvise MADV_DOEXEC

On 16.08.21 14:27, 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.
> 
> 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).
> 

Well, that comment did turn out not-so-private, so it's certainly a good 
discussion-starter.

-- 
Thanks,

David / dhildenb