From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail191.messagelabs.com (mail191.messagelabs.com [216.82.242.19]) by kanga.kvack.org (Postfix) with ESMTP id B1C7A6B01EE for ; Tue, 6 Apr 2010 01:09:32 -0400 (EDT) Subject: mprotect pgprot handling weirdness From: Benjamin Herrenschmidt Content-Type: text/plain; charset="UTF-8" Date: Tue, 06 Apr 2010 15:09:26 +1000 Message-ID: <1270530566.13812.28.camel@pasglop> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org To: linux-mm@kvack.org Cc: "linux-kernel@vger.kernel.org" List-ID: Hi folks ! While looking at untangling a bit some of the mess with vm_flags and pgprot (*), I notices a few things I can't quite explain... they may .. or may not be bugs, but I though it was worth mentioning: - In mprotect_fixup() : /* * vm_flags and vm_page_prot are protected by the mmap_sem * held in write mode. */ vma->vm_flags = newflags; vma->vm_page_prot = pgprot_modify(vma->vm_page_prot, vm_get_page_prot(newflags)); if (vma_wants_writenotify(vma)) { vma->vm_page_prot = vm_get_page_prot(newflags & ~VM_SHARED); dirty_accountable = 1; } So as you can see above, we take great care (using pgprot_modify) to avoid blasting away some PAT related flags on x86 (no other arch implements pgprot_modify() today).... but if we hit vma_wants_writenotify(), then we unconditionally override the entire vma->vm_page_prot field with some new prot bits born of the new vm_flags. That sounds odd... - in sys_mprotect: newflags = vm_flags | (vma->vm_flags & ~(VM_READ | VM_WRITE | VM_EXEC)); Do I read correctly that this means we cannot -remove- any flag than VM_READ, VM_WRITE or VM_EXEC ? That means that we cannot remove PROT_SAO which gets turned into VM_SAO on powerpc ... Yet another reason to take those arch specific mapping attributes out of the vm_flags. (*) Right now it's near impossible to add arch specific PROT_* bits to mmap/mprotect for fancy things like cachability attributes, or other nifty things like reverse-endian mappings that we have on some embedded platforms, I'm investigating ways to better separate vm_page_prot from vm_flags so some PROT_* bits can go straight to the former without having to be mirrored in some way in the later. Cheers, Ben. -- 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