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

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Date:   Wed, 23 May 2018 10:46:26 -0700
From:   Nadav Amit <nadav.amit@...il.com>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        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

Dave Hansen <dave.hansen@...ux.intel.com> wrote:

> On 05/22/2018 10:51 AM, Matthew Wilcox wrote:
>> 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.
> 
> Yep, and you'd probably want a cache of them so you don't end up having
> to go rewrite half of the PGD every time you context-switch.  But, on
> the plus side, the logic would be pretty similar if not identical to the
> way that we manage PCIDs.  If your mm was recently active on the CPU,
> you can use a PGD that's already been constructed.  If not, you're stuck
> making a new one.
> 
> Andy L. was alto talking about using this kind of mechanism to simplify
> the entry code.  Instead of needing per-cpu areas where we index by the
> CPU number, or by using %GS, we could have per-cpu data or code that has
> a fixed virtual address.
> 
> It'd be a fun project, but it might not ever pan out.

For the record: there are several academic studies about this subject. The
most notable one is Corey [1].

[1] https://www.usenix.org/legacy/event/osdi08/tech/full_papers/boyd-wickizer/boyd_wickizer.pdf