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Date:   Mon, 11 Apr 2022 11:47:20 -0700
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To:     Khalid Aziz <khalid.aziz@oracle.com>, akpm@linux-foundation.org,
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References: <cover.1649370874.git.khalid.aziz@oracle.com>
From:   Dave Hansen <dave.hansen@intel.com>
Subject: Re: [PATCH v1 00/14] Add support for shared PTEs across processes
In-Reply-To: <cover.1649370874.git.khalid.aziz@oracle.com>
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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?