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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