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

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Date:   Tue, 29 Oct 2019 09:56:02 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        Mike Rapoport <rppt@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Arnd Bergmann <arnd@...db.de>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        James Bottomley <jejb@...ux.ibm.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Linux API <linux-api@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Mike Rapoport <rppt@...ux.ibm.com>
Subject: Re: [PATCH RFC] mm: add MAP_EXCLUSIVE to create exclusive user
 mappings

On Tue, Oct 29, 2019 at 09:43:18AM +0300, Kirill A. Shutemov wrote:
> But some CPUs don't like to have two TLB entries for the same memory with
> different sizes at the same time. See for instance AMD erratum 383.
> 
> Getting it right would require making the range not present, flush TLB and
> only then install huge page. That's what we do for userspace.
> 
> It will not fly for the direct mapping. There is no reasonable way to
> exclude other CPU from accessing the range while it's not present (call
> stop_machine()? :P). Moreover, the range may contain the code that doing
> the collapse or data required for it...
> 
> BTW, looks like current __split_large_page() in pageattr.c is susceptible
> to the errata. Maybe we can get away with the easy way...

As you write above, there is just no way we can have a (temporary) hole
in the direct map.

We are careful about that other errata, and make sure both translations
are identical wrt everything else.