linux-kernel - Re: [RFC PATCH 02/14] sparc64: add new fields to mmu context for shared context support

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Message-ID: <20161221181243.GB3311@ravnborg.org>
Date:   Wed, 21 Dec 2016 19:12:43 +0100
From:   Sam Ravnborg <sam@...nborg.org>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     sparclinux@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        "David S . Miller" <davem@...emloft.net>,
        Bob Picco <bob.picco@...cle.com>,
        Nitin Gupta <nitin.m.gupta@...cle.com>,
        Vijay Kumar <vijay.ac.kumar@...cle.com>,
        Julian Calaby <julian.calaby@...il.com>,
        Adam Buchbinder <adam.buchbinder@...il.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Michal Hocko <mhocko@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC PATCH 02/14] sparc64: add new fields to mmu context for
 shared context support

Hi Mike.

On Sun, Dec 18, 2016 at 03:33:59PM -0800, Mike Kravetz wrote:
> On 12/16/2016 11:34 PM, Sam Ravnborg wrote:
> > Hi Mike.
> > 
> > On Fri, Dec 16, 2016 at 10:35:25AM -0800, Mike Kravetz wrote:
> >> Add new fields to the mm_context structure to support shared context.
> >> Instead of a simple context ID, add a pointer to a structure with a
> >> reference count.  This is needed as multiple tasks will share the
> >> context ID.
> > 
> > What are the benefits with the shared_mmu_ctx struct?
> > It does not save any space in mm_context_t, and the CPU only
> > supports one extra context.
> > So it looks like over-engineering with all the extra administration
> > required to handle it with refcount, poitners etc.
> > 
> > what do I miss?
> 
> Multiple tasks will share this same context ID.  The first task to need
> a new shared context will allocate the structure, increment the ref count
> and point to it.  As other tasks join the sharing, they will increment
> the ref count and point to the same structure.  Similarly, when tasks
> no longer use the shared context ID, they will decrement the reference
> count.
> 
> The reference count is important so that we will know when the last
> reference to the shared context ID is dropped.  When the last reference
> is dropped, then the ID can be recycled/given back to the global pool
> of context IDs.
> 
> This seemed to be the most straight forward way to implement this.

This nice explanation clarified it - thanks.
Could you try to include this info in the description
of the struct - so it is obvious what the intention with the
reference counter is.

	Sam