From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-bk0-f45.google.com (mail-bk0-f45.google.com [209.85.214.45]) by kanga.kvack.org (Postfix) with ESMTP id 743396B0069 for ; Tue, 1 Apr 2014 17:22:08 -0400 (EDT) Received: by mail-bk0-f45.google.com with SMTP id na10so1337836bkb.4 for ; Tue, 01 Apr 2014 14:22:07 -0700 (PDT) Received: from zene.cmpxchg.org (zene.cmpxchg.org. [2a01:238:4224:fa00:ca1f:9ef3:caee:a2bd]) by mx.google.com with ESMTPS id nt9si66448bkb.319.2014.04.01.14.22.06 for (version=TLSv1 cipher=RC4-SHA bits=128/128); Tue, 01 Apr 2014 14:22:06 -0700 (PDT) Date: Tue, 1 Apr 2014 17:21:02 -0400 From: Johannes Weiner Subject: Re: [PATCH 0/5] Volatile Ranges (v12) & LSF-MM discussion fodder Message-ID: <20140401212102.GM4407@cmpxchg.org> References: <1395436655-21670-1-git-send-email-john.stultz@linaro.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1395436655-21670-1-git-send-email-john.stultz@linaro.org> Sender: owner-linux-mm@kvack.org List-ID: To: John Stultz Cc: LKML , Andrew Morton , Android Kernel Team , Robert Love , Mel Gorman , Hugh Dickins , Dave Hansen , Rik van Riel , Dmitry Adamushko , Neil Brown , Andrea Arcangeli , Mike Hommey , Taras Glek , Jan Kara , KOSAKI Motohiro , Michel Lespinasse , Minchan Kim , "H. Peter Anvin" , "linux-mm@kvack.org" [ I tried to bring this up during LSFMM but it got drowned out. Trying again :) ] On Fri, Mar 21, 2014 at 02:17:30PM -0700, John Stultz wrote: > Optimistic method: > 1) Userland marks a large range of data as volatile > 2) Userland continues to access the data as it needs. > 3) If userland accesses a page that has been purged, the kernel will > send a SIGBUS > 4) Userspace can trap the SIGBUS, mark the affected pages as > non-volatile, and refill the data as needed before continuing on As far as I understand, if a pointer to volatile memory makes it into a syscall and the fault is trapped in kernel space, there won't be a SIGBUS, the syscall will just return -EFAULT. Handling this would mean annotating every syscall invocation to check for -EFAULT, refill the data, and then restart the syscall. This is complicated even before taking external libraries into account, which may not propagate syscall returns properly or may not be reentrant at the necessary granularity. Another option is to never pass volatile memory pointers into the kernel, but that too means that knowledge of volatility has to travel alongside the pointers, which will either result in more complexity throughout the application or severely limited scope of volatile memory usage. Either way, optimistic volatile pointers are nowhere near as transparent to the application as the above description suggests, which makes this usecase not very interesting, IMO. If we can support it at little cost, why not, but I don't think we should complicate the common usecases to support this one. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org