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Date:   Fri, 18 May 2018 14:14:20 +0000
From:   Christopher Lameter <cl@linux.com>
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,
        Peter Zijlstra <peterz@infradead.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
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On Tue, 15 May 2018, Boaz Harrosh wrote:

> > I don't think page tables work the way you think they work.
> >
> > +               err = vm_insert_pfn_prot(zt->vma, zt_addr, pfn, prot);
> >
> > That doesn't just insert it into the local CPU's page table.  Any CPU
> > which directly accesses or even prefetches that address will also get
> > the translation into its cache.
> >
>
> 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.

But there are no provisions for probhiting accesses from other cores?

This means that a casual accidental write from a thread executing on
another core can lead to arbitrary memory corruption because the cache
flushing has been bypassed.