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

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Message-ID: <20191029110024.yjytp22lhd2vekrv@box>
Date:   Tue, 29 Oct 2019 14:00:24 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Peter Zijlstra <peterz@...radead.org>
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 09:56:02AM +0100, Peter Zijlstra wrote:
> 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.

It's not clear if it is enough to avoid the issue. "under a highly specific
and detailed set of conditions" is not very specific set of conditions :P

-- 
 Kirill A. Shutemov