linux-kernel - Re: [PATCH] mm: Add new vma flag VM_LOCAL

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Date:   Tue, 22 May 2018 10:51:14 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     Christopher Lameter <cl@...ux.com>,
        Boaz Harrosh <boazh@...app.com>,
        Jeff Moyer <jmoyer@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Rik van Riel <riel@...hat.com>, Jan Kara <jack@...e.cz>,
        Matthew Wilcox <mawilcox@...rosoft.com>,
        Amit Golander <Amit.Golander@...app.com>
Subject: Re: [PATCH] mm: Add new vma flag VM_LOCAL_CPU

On Tue, May 22, 2018 at 10:03:54AM -0700, Dave Hansen wrote:
> On 05/22/2018 09:46 AM, Christopher Lameter wrote:
> > On Tue, 22 May 2018, Dave Hansen wrote:
> > 
> >> On 05/22/2018 09:05 AM, Boaz Harrosh wrote:
> >>> How can we implement "Private memory"?
> >> Per-cpu page tables would do it.
> > We already have that for percpu subsystem. See alloc_percpu()
> 
> I actually mean a set of page tables which is only ever installed on a
> single CPU.  The CPU is architecturally allowed to go load any PTE in
> the page tables into the TLB any time it feels like.  The only way to
> keep a PTE from getting into the TLB is not ensure that a CPU never has
> any access to it, and the only way to do that is to make sure that no
> set of page tables it ever loads into CR3 have that PTE.
> 
> As Peter said, it's possible, but not pretty.

But CR3 is a per-CPU register.  So it'd be *possible* to allocate one
PGD per CPU (per process).  Have them be identical in all but one of
the PUD entries.  Then you've reserved 1/512 of your address space for
per-CPU pages.

Complicated, ugly, memory-consuming.  But possible.