From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail138.messagelabs.com (mail138.messagelabs.com [216.82.249.35]) by kanga.kvack.org (Postfix) with SMTP id 348576B0087 for ; Fri, 3 Dec 2010 11:28:52 -0500 (EST) Received: by pzk27 with SMTP id 27so1964070pzk.14 for ; Fri, 03 Dec 2010 08:28:27 -0800 (PST) Date: Sat, 4 Dec 2010 01:28:17 +0900 From: Minchan Kim Subject: Re: Flushing whole page instead of work for ptrace Message-ID: <20101203162817.GA21438@barrios-desktop> References: <4CEFA8AE.2090804@petalogix.com> <20101130233250.35603401C8@magilla.sf.frob.com> <20101203150021.GA11114@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20101203150021.GA11114@redhat.com> Sender: owner-linux-mm@kvack.org To: Oleg Nesterov Cc: Roland McGrath , michal.simek@petalogix.com, Andrew Morton , LKML , linux-mm@kvack.org, John Williams , "Edgar E. Iglesias" , Hugh Dickins , Nick Piggin List-ID: On Fri, Dec 03, 2010 at 04:00:21PM +0100, Oleg Nesterov wrote: > On 11/30, Roland McGrath wrote: > > > > Documentation/cachetlb.txt says: > > > > Any time the kernel writes to a page cache page, _OR_ > > the kernel is about to read from a page cache page and > > user space shared/writable mappings of this page potentially > > exist, this routine is called. > > > > In your case, the kernel is only reading (write=0 passed to > > access_process_vm and get_user_pages). In normal situations, > > the page in question will have only a private and read-only > > mapping in user space. So the call should not be required in > > these cases--if the code can tell that's so. > > > > Perhaps something like the following would be safe. > > But you really need some VM folks to tell you for sure. > > > > diff --git a/mm/memory.c b/mm/memory.c > > index 02e48aa..2864ee7 100644 > > --- a/mm/memory.c > > +++ b/mm/memory.c > > @@ -1484,7 +1484,8 @@ int __get_user_pages(struct task_struct *tsk, struct mm_struct *mm, > > pages[i] = page; > > > > flush_anon_page(vma, page, start); > > - flush_dcache_page(page); > > + if ((vm_flags & VM_WRITE) || (vma->vm_flags & VM_SHARED) > > + flush_dcache_page(page); > > First of all, I know absolutely nothing about D-cache aliasing. > My poor understanding of flush_dcache_page() is: synchronize the > kernel/user vision of this memory, in the case when either side > can change it. > > If this is true, then this change doesn't look right in general. > > Even if (vma->vm_flags & VM_SHARED) == 0, it is possible that > tsk can write to this memory, this mapping can be writable and > private. > > Even if we ensure that this mapping is readonly/private, another > user-space process can write to this page via shared/writable > mapping. > I think you're right. It has a portential that other processes have a such mapping. > > I'd like to know if my understanding is correct, I am just curious. > > Oleg. How about this? Maybe this patch would mitigate the overhead. But I am not sure this patch. Cced GUP experts.