Re: [PATCH] mm: Add new vma flag VM_LOCAL_CPU

From: Matthew Wilcox <willy@infradead.org>
To: Boaz Harrosh <boazh@netapp.com>
Cc: 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
Date: Mon, 14 May 2018 17:41:37 -0700	[thread overview]
Message-ID: <20180515004137.GA5168@bombadil.infradead.org> (raw)
In-Reply-To: <7ec6fa37-8529-183d-d467-df3642bcbfd2@netapp.com>

On Mon, May 14, 2018 at 10:37:38PM +0300, Boaz Harrosh wrote:
> On 14/05/18 22:15, Matthew Wilcox wrote:
> > On Mon, May 14, 2018 at 08:28:01PM +0300, Boaz Harrosh wrote:
> >> On a call to mmap an mmap provider (like an FS) can put
> >> this flag on vma->vm_flags.
> >>
> >> The VM_LOCAL_CPU flag tells the Kernel that the vma will be used
> >> from a single-core only, and therefore invalidation (flush_tlb) of
> >> PTE(s) need not be a wide CPU scheduling.
> > 
> > I still don't get this.  You're opening the kernel up to being exploited
> > by any application which can persuade it to set this flag on a VMA.
> > 
> 
> No No this is not an application accessible flag this can only be set
> by the mmap implementor at ->mmap() time (Say same as VM_VM_MIXEDMAP).
> 
> Please see the zuf patches for usage (Again apologise for pushing before
> a user)
> 
> The mmap provider has all the facilities to know that this can not be
> abused, not even by a trusted Server.

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.