linux-kernel - Re: [PATCH RFC] mm: add MAP_EXCLUSIVE to create exclusive user mappings

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Date:   Wed, 30 Oct 2019 17:48:20 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To:     "peterz@...radead.org" <peterz@...radead.org>
CC:     "adobriyan@...il.com" <adobriyan@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "rppt@...nel.org" <rppt@...nel.org>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        "jejb@...ux.ibm.com" <jejb@...ux.ibm.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "hpa@...or.com" <hpa@...or.com>,
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        "bp@...en8.de" <bp@...en8.de>,
        "rppt@...ux.ibm.com" <rppt@...ux.ibm.com>,
        "arnd@...db.de" <arnd@...db.de>
Subject: Re: [PATCH RFC] mm: add MAP_EXCLUSIVE to create exclusive user
 mappings

On Wed, 2019-10-30 at 11:04 +0100, Peter Zijlstra wrote:
> > You mean shatter performance?
> 
> Shatter (all) large pages.

So it looks like this is already happening then to some degree. It's not just
BPF either, any module_alloc() user is going to do something similar with the
direct map alias of the page they got for the text.

So there must be at least some usages where breaking the direct map down, for
like a page to store a key or something, isn't totally horrible.