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Date:   Tue, 15 May 2018 13:47:55 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Boaz Harrosh <boazh@netapp.com>
Cc:     Matthew Wilcox <willy@infradead.org>,
        Jeff Moyer <jmoyer@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>, x86@kernel.org,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Rik van Riel <riel@redhat.com>, Jan Kara <jack@suse.cz>,
        Matthew Wilcox <mawilcox@microsoft.com>,
        Amit Golander <Amit.Golander@netapp.com>
Subject: Re: [PATCH] mm: Add new vma flag VM_LOCAL_CPU
Message-ID: <20180515114755.GY12217@hirez.programming.kicks-ass.net>
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On Tue, May 15, 2018 at 01:43:23PM +0300, Boaz Harrosh wrote:
> Yes I know, but that is exactly the point of this flag. I know that this
> address is only ever accessed from a single core. Because it is an mmap (vma)
> of an O_TMPFILE-exclusive file created in a core-pinned thread and I allow
> only that thread any kind of access to this vma. Both the filehandle and the
> mmaped pointer are kept on the thread stack and have no access from outside.
> 
> So the all point of this flag is the kernel driver telling mm that this
> address is enforced to only be accessed from one core-pinned thread.

What happens when the userspace part -- there is one, right, how else do
you get an mm to stick a vma in -- simply does a full address range
probe scan?

Something like this really needs a far more detailed Changelog that
explains how its to be used and how it is impossible to abuse. Esp. that
latter is _very_ important.