linux-kernel - Re: [RFC PATCH 0/6] Add support for shared PTEs across processes

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Message-ID: <Yec1OZ0reX2EMXL7@casper.infradead.org>
Date:   Tue, 18 Jan 2022 21:46:33 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Khalid Aziz <khalid.aziz@...cle.com>, akpm@...ux-foundation.org,
        longpeng2@...wei.com, arnd@...db.de, dave.hansen@...ux.intel.com,
        david@...hat.com, rppt@...nel.org, surenb@...gle.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC PATCH 0/6] Add support for shared PTEs across processes

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.