From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <47E62DBA.4050102@qumranet.com> Date: Sun, 23 Mar 2008 12:15:22 +0200 From: Avi Kivity MIME-Version: 1.0 Subject: Re: [kvm-devel] [RFC/PATCH 01/15] preparation: provide hook to enable pgstes in user pagetable References: <1206028710.6690.21.camel@cotte.boeblingen.de.ibm.com> <1206030278.6690.52.camel@cotte.boeblingen.de.ibm.com> <47E29EC6.5050403@goop.org> <1206040405.8232.24.camel@nimitz.home.sr71.net> <47E2CAAC.6020903@de.ibm.com> <1206124176.30471.27.camel@nimitz.home.sr71.net> <20080322175705.GD6367@osiris.boeblingen.de.ibm.com> In-Reply-To: <20080322175705.GD6367@osiris.boeblingen.de.ibm.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org Return-Path: To: Heiko Carstens Cc: Dave Hansen , Jeremy Fitzhardinge , Christian Ehrhardt , hollisb@us.ibm.com, arnd@arndb.de, Linux Memory Management List , carsteno@de.ibm.com, heicars2@linux.vnet.ibm.com, mschwid2@linux.vnet.ibm.com, jeroney@us.ibm.com, borntrae@linux.vnet.ibm.com, virtualization@lists.linux-foundation.org, kvm-devel@lists.sourceforge.net, rvdheij@gmail.com, Olaf Schnapper , jblunck@suse.de, "Zhang, Xiantao" List-ID: Heiko Carstens wrote: >> What you've done with dup_mm() is probably the brute-force way that I >> would have done it had I just been trying to make a proof of concept or >> something. I'm worried that there are a bunch of corner cases that >> haven't been considered. >> >> What if someone else is poking around with ptrace or something similar >> and they bump the mm_users: >> >> + if (tsk->mm->context.pgstes) >> + return 0; >> + if (!tsk->mm || atomic_read(&tsk->mm->mm_users) > 1 || >> + tsk->mm != tsk->active_mm || tsk->mm->ioctx_list) >> + return -EINVAL; >> -------->HERE >> + tsk->mm->context.pgstes = 1; /* dirty little tricks .. */ >> + mm = dup_mm(tsk); >> >> It'll race, possibly fault in some other pages, and those faults will be >> lost during the dup_mm(). I think you need to be able to lock out all >> of the users of access_process_vm() before you go and do this. You also >> need to make sure that anyone who has looked at task->mm doesn't go and >> get a reference to it and get confused later when it isn't the task->mm >> any more. >> >> >>> Therefore, we need to reallocate the page table after fork() >>> once we know that task is going to be a hypervisor. That's what this >>> code does: reallocate a bigger page table to accomondate the extra >>> information. The task needs to be single-threaded when calling for >>> extended page tables. >>> >>> Btw: at fork() time, we cannot tell whether or not the user's going to >>> be a hypervisor. Therefore we cannot do this in fork. >>> >> Can you convert the page tables at a later time without doing a >> wholesale replacement of the mm? It should be a bit easier to keep >> people off the pagetables than keep their grubby mitts off the mm >> itself. >> > > Yes, as far as I can see you're right. And whatever we do in arch code, > after all it's just a work around to avoid a new clone flag. > If something like clone() with CLONE_KVM would be useful for more > architectures than just s390 then maybe we should try to get a flag. > > Oh... there are just two unused clone flag bits left. Looks like the > namespace changes ate up a lot of them lately. > > Well, we could still play dirty tricks like setting a bit in current > via whatever mechanism which indicates child-wants-extended-page-tables > and then just fork and be happy. > How about taking mmap_sem for write and converting all page tables in-place? I'd rather avoid the need to fork() when creating a VM. -- error compiling committee.c: too many arguments to function -- 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