linux-kernel - Re: [PATCH v1 00/14] Add support for shared PTEs across processes

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Date:   Mon, 11 Apr 2022 11:47:20 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Khalid Aziz <khalid.aziz@...cle.com>, akpm@...ux-foundation.org,
        willy@...radead.org
Cc:     aneesh.kumar@...ux.ibm.com, arnd@...db.de, 21cnbao@...il.com,
        corbet@....net, dave.hansen@...ux.intel.com, david@...hat.com,
        ebiederm@...ssion.com, hagen@...u.net, jack@...e.cz,
        keescook@...omium.org, kirill@...temov.name, kucharsk@...il.com,
        linkinjeon@...nel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        longpeng2@...wei.com, luto@...nel.org, markhemm@...glemail.com,
        pcc@...gle.com, rppt@...nel.org, sieberf@...zon.com,
        sjpark@...zon.de, surenb@...gle.com, tst@...oebel-theuer.de,
        yzaikin@...gle.com
Subject: Re: [PATCH v1 00/14] Add support for shared PTEs across processes

On 4/11/22 09:05, Khalid Aziz wrote:
> PTEs are shared at pgdir level and hence it imposes following
> requirements on the address and size given to the mshare():
> 
> - Starting address must be aligned to pgdir size (512GB on x86_64).
>   This alignment value can be looked up in /proc/sys/vm//mshare_size
> - Size must be a multiple of pgdir size
> - Any mappings created in this address range at any time become
>   shared automatically
> - Shared address range can have unmapped addresses in it. Any access
>   to unmapped address will result in SIGBUS
> 
> 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.

What does this mean in practice?