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Date: Wed, 02 Apr 2014 12:51:45 -0700 From: John Stultz <john.stultz@...aro.org> To: Andrea Arcangeli <aarcange@...hat.com> CC: Johannes Weiner <hannes@...xchg.org>, LKML <linux-kernel@...r.kernel.org>, Andrew Morton <akpm@...ux-foundation.org>, Android Kernel Team <kernel-team@...roid.com>, Robert Love <rlove@...gle.com>, Mel Gorman <mel@....ul.ie>, Hugh Dickins <hughd@...gle.com>, Dave Hansen <dave@...1.net>, Rik van Riel <riel@...hat.com>, Dmitry Adamushko <dmitry.adamushko@...il.com>, Neil Brown <neilb@...e.de>, Mike Hommey <mh@...ndium.org>, Taras Glek <tglek@...illa.com>, Jan Kara <jack@...e.cz>, KOSAKI Motohiro <kosaki.motohiro@...il.com>, Michel Lespinasse <walken@...gle.com>, Minchan Kim <minchan@...nel.org>, "H. Peter Anvin" <hpa@...or.com>, "linux-mm@...ck.org" <linux-mm@...ck.org> Subject: Re: [PATCH 0/5] Volatile Ranges (v12) & LSF-MM discussion fodder On 04/02/2014 11:31 AM, Andrea Arcangeli wrote: > On Tue, Apr 01, 2014 at 09:03:57PM -0700, John Stultz wrote: >> Now... once you've chosen SIGBUS semantics, there will be folks who will >> try to exploit the fact that we get SIGBUS on purged page access (at >> least on the user-space side) and will try to access pages that are >> volatile until they are purged and try to then handle the SIGBUS to fix >> things up. Those folks exploiting that will have to be particularly >> careful not to pass volatile data to the kernel, and if they do they'll >> have to be smart enough to handle the EFAULT, etc. That's really all >> their problem, because they're being clever. :) > I'm actually working on feature that would solve the problem for the > syscalls accessing missing volatile pages. So you'd never see a > -EFAULT because all syscalls won't return even if they encounters a > missing page in the volatile range dropped by the VM pressure. > > It's called userfaultfd. You call sys_userfaultfd(flags) and it > connects the current mm to a pseudo filedescriptor. The filedescriptor > works similarly to eventfd but with a different protocol. So yea! I actually think (its been awhile now) I mentioned your work to Taras (or maybe he mentioned it to me?), but it did seem like the userfaltfd would be a better solution for the style of fault handling they were thinking about. (Especially as actually handling SIGBUS and doing something sane in a large threaded application seems very difficult). That said, explaining volatile ranges as a concept has been difficult enough without mixing in other new concepts :), so I'm hesitant to tie the functionality together in until its clear the userfaultfd approach is likely to land. But maybe I need to take a closer look at it. thanks -john -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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