Date: Wed, 15 Nov 2017 08:20:57 -0800 From: Tycho Andersen <tycho@...ho.ws> To: Matthew Wilcox <willy@...radead.org> Cc: Dave Hansen <dave.hansen@...el.com>, linux-kernel@...r.kernel.org, linux-mm@...ck.org, kernel-hardening@...ts.openwall.com, Marco Benatto <marco.antonio.780@...il.com>, Juerg Haefliger <juerg.haefliger@...onical.com>, x86@...nel.org Subject: Re: Re: [PATCH v6 03/11] mm, x86: Add support for eXclusive Page Frame Ownership (XPFO) On Wed, Nov 15, 2017 at 06:58:35AM -0800, Matthew Wilcox wrote: > On Tue, Nov 14, 2017 at 11:00:20PM -0800, Dave Hansen wrote: > > On 11/14/2017 07:44 PM, Matthew Wilcox wrote: > > > We don't need to kmap in order to access MOVABLE allocations. kmap is > > > only needed for HIGHMEM allocations. So there's nothing wrong with ext4 > > > or set_bh_page(). > > > > Yeah, it's definitely not _buggy_. > > > > Although, I do wonder what we should do about these for XPFO. Should we > > just stick a kmap() in there and comment it? What we really need is a > > mechanism to say "use this as a kernel page" and "stop using this as a > > kernel page". kmap() does that... kinda. It's not a perfect fit, but > > it's pretty close. > > It'd be kind of funny if getting XPFO working better means improving > how well Linux runs on 32-bit machines with HIGHMEM. I think there's > always going to be interest in those -- ARM developed 36 bit physmem > before biting the bullet and going to arm64. Maybe OpenRISC will do > that next ;-) Oh, sorry, I didn't realize that this wasn't a bug. In any case, this seems like sort of an uphill battle -- lots of places are going to do stuff like this since it's legal, adding code to work around it just for XPFO seems like a lot of burden on the kernel. (Of course, I'm open to convincing :) How common are these MOVABLE allocations that the kernel does? What if we did some hybrid approach, where we re-map the lists based on MOVABLE/UNMOVABLE, but then check the actual GFP flags on allocation to see if they match what we set when populating the free list, and re-map accordingly if they don't. Or is there some other way? Cheers, Tycho
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