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Date:   Tue, 18 Jan 2022 21:46:33 +0000
From:   Matthew Wilcox <willy@infradead.org>
To:     Dave Hansen <dave.hansen@intel.com>
Cc:     Khalid Aziz <khalid.aziz@oracle.com>, akpm@linux-foundation.org,
        longpeng2@huawei.com, arnd@arndb.de, dave.hansen@linux.intel.com,
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        linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [RFC PATCH 0/6] Add support for shared PTEs across processes
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References: <cover.1642526745.git.khalid.aziz@oracle.com>
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On Tue, Jan 18, 2022 at 01:41:40PM -0800, Dave Hansen wrote:
> On 1/18/22 1:19 PM, Khalid Aziz wrote:
> > - Starting address must be aligned to pgdir size (512GB on x86_64)
> 
> How does this work on systems with 5-level paging where a top-level page
> table entry covers 256TB?  Is the alignment requirement 512GB or 256TB?
>  How does userspace figure out which alignment requirement it is subject to?

The original idea was any power of two, naturally aligned, >= PAGE_SIZE,
but I suspect Khalid has simplified it for this first implementation.