From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail143.messagelabs.com (mail143.messagelabs.com [216.82.254.35]) by kanga.kvack.org (Postfix) with ESMTP id 79C866B0055 for ; Mon, 23 Feb 2009 02:30:21 -0500 (EST) Message-ID: <49A25086.30606@goop.org> Date: Sun, 22 Feb 2009 23:30:14 -0800 From: Jeremy Fitzhardinge MIME-Version: 1.0 Subject: Re: [PATCH RFC] vm_unmap_aliases: allow callers to inhibit TLB flush References: <49416494.6040009@goop.org> <200902200441.08541.nickpiggin@yahoo.com.au> <499DAEE4.8010507@goop.org> <200902231514.01965.nickpiggin@yahoo.com.au> In-Reply-To: <200902231514.01965.nickpiggin@yahoo.com.au> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org To: Nick Piggin Cc: Andrew Morton , Linux Kernel Mailing List , Linux Memory Management List , the arch/x86 maintainers , Arjan van de Ven List-ID: Nick Piggin wrote: > On Friday 20 February 2009 06:11:32 Jeremy Fitzhardinge wrote: > >> Nick Piggin wrote: >> >>> Then what is the point of the vm_unmap_aliases? If you are doing it >>> for security it won't work because other CPUs might still be able >>> to write through dangling TLBs. If you are not doing it for >>> security then it does not need to be done at all. >>> >> Xen will make sure any danging tlb entries are flushed before handing >> the page out to anyone else. >> >> >>> Unless it is something strange that Xen does with the page table >>> structure and you just need to get rid of those? >>> >> Yeah. A pte pointing at a page holds a reference on it, saying that it >> belongs to the domain. You can't return it to Xen until the refcount is 0. >> > > OK. Then I will remember to find some time to get the interrupt > safe patches working. I wonder why you can't just return it to > Xen when (or have Xen hold it somewhere until) the refcount > reaches 0? > It would still need to allocate a page in the meantime, which could fail because the domain has hit its hard memory limit (which will be the common case, because a domain generally starts with its full compliment of memory). The nice thing about the exchange is that there's no accounting to take into account. >>> Or... what if we just allow a compile and/or boot time flag to direct >>> that it does not want lazy vmap unmapping and it will just revert to >>> synchronous unmapping? If Xen needs lots of flushing anyway it might >>> not be a win anyway. >>> >> That may be worth considering. >> > > ... in the meantime, shall we just do this for Xen? It is probably > safer and may end up with no worse performance on Xen anyway. If > we get more vmap users and it becomes important, you could look at > more sophisticated ways of doing this. Eg. a page could be flagged > if it potentially has lazy vmaps. > OK. Do you want to do the patch, or shall I? J -- 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