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Message-ID: <20180214135141.GA16215@bombadil.infradead.org>
Date: Wed, 14 Feb 2018 05:51:41 -0800
From: Matthew Wilcox <willy@...radead.org>
To: "Kirill A. Shutemov" <kirill@...temov.name>
Cc: Daniel Micay <danielmicay@...il.com>, Jann Horn <jannh@...gle.com>,
linux-mm@...ck.org,
Kernel Hardening <kernel-hardening@...ts.openwall.com>,
kernel list <linux-kernel@...r.kernel.org>,
"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [RFC] Limit mappings to ten per page per process
On Fri, Feb 09, 2018 at 07:26:09AM +0300, Kirill A. Shutemov wrote:
> On Thu, Feb 08, 2018 at 01:37:43PM -0800, Matthew Wilcox wrote:
> > On Thu, Feb 08, 2018 at 12:21:00PM -0800, Matthew Wilcox wrote:
> > > Now that I think about it, though, perhaps the simplest solution is not
> > > to worry about checking whether _mapcount has saturated, and instead when
> > > adding a new mmap, check whether this task already has it mapped 10 times.
> > > If so, refuse the mapping.
> >
> > That turns out to be quite easy. Comments on this approach?
>
> This *may* break some remap_file_pages() users.
We have some?! ;-) I don't understand the use case where they want to
map the same page of a file multiple times into the same process. I mean,
yes, of course, they might ask for it, but I don't understand why they would.
Do you have any insight here?
> And it may be rather costly for popular binaries. Consider libc.so.
We already walk this tree to insert the mapping; this just adds a second
walk of the tree to check which overlapping mappings exist. I would
expect it to just make the tree cache-hot.
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