From mboxrd@z Thu Jan  1 00:00:00 1970
From: Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH RFC] mm: add MAP_EXCLUSIVE to create exclusive user
 mappings
Date: Mon, 28 Oct 2019 14:55:21 +0100
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To: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Mike Rapoport <rppt@kernel.org>, linux-kernel@vger.kernel.org, Alexey Dobriyan <adobriyan@gmail.com>, Andrew Morton <akpm@linux-foundation.org>, Andy Lutomirski <luto@kernel.org>, Arnd Bergmann <arnd@arndb.de>, Borislav Petkov <bp@alien8.de>, Dave Hansen <dave.hansen@linux.intel.com>, James Bottomley <jejb@linux.ibm.com>, Steven Rostedt <rostedt@goodmis.org>, Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>, linux-api@vger.kernel.org, linux-mm@kvack.org, x86@kernel.org, Mike Rapoport <rppt@linux.ibm.com>
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On Mon, Oct 28, 2019 at 04:16:23PM +0300, Kirill A. Shutemov wrote:

> I think active use of this feature will lead to performance degradation of
> the system with time.
> 
> Setting a single 4k page non-present in the direct mapping will require
> splitting 2M or 1G page we usually map direct mapping with. And it's one
> way road. We don't have any mechanism to map the memory with huge page
> again after the application has freed the page.

Right, we recently had a 'bug' where ftrace triggered something like
this and facebook ran into it as a performance regression. So yes, this
is a real concern.