linux-kernel - Re: [PATCH 10/17] prmem: documentation

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Message-ID: <20181030212551.GD10491@bombadil.infradead.org>
Date:   Tue, 30 Oct 2018 14:25:51 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     nadav.amit@...il.com, Kees Cook <keescook@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Igor Stoppa <igor.stoppa@...il.com>,
        Mimi Zohar <zohar@...ux.vnet.ibm.com>,
        Dave Chinner <david@...morbit.com>,
        James Morris <jmorris@...ei.org>,
        Michal Hocko <mhocko@...nel.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        linux-integrity <linux-integrity@...r.kernel.org>,
        linux-security-module <linux-security-module@...r.kernel.org>,
        Igor Stoppa <igor.stoppa@...wei.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Jonathan Corbet <corbet@....net>,
        Laura Abbott <labbott@...hat.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 10/17] prmem: documentation

On Tue, Oct 30, 2018 at 11:51:17AM -0700, Andy Lutomirski wrote:
> Finally, one issue: rare_alloc() is going to utterly suck
> performance-wise due to the global IPI when the region gets zapped out
> of the direct map or otherwise made RO.  This is the same issue that
> makes all existing XPO efforts so painful. We need to either optimize
> the crap out of it somehow or we need to make sure it’s not called
> except during rare events like device enumeration.

Batching operations is kind of the whole point of the VM ;-)  Either
this rare memory gets used a lot, in which case we'll want to create slab
caches for it, make it a MM zone and the whole nine yeards, or it's not
used very much in which case it doesn't matter that performance sucks.

For now, I'd suggest allocating 2MB chunks as needed, and having a
shrinker to hand back any unused pieces.