From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail144.messagelabs.com (mail144.messagelabs.com [216.82.254.51]) by kanga.kvack.org (Postfix) with ESMTP id B82CA6B004F for ; Tue, 21 Jul 2009 03:16:54 -0400 (EDT) Date: Tue, 21 Jul 2009 09:16:54 +0200 From: Nick Piggin Subject: Re: [PATCH 05/10] ksm: no debug in page_dup_rmap() Message-ID: <20090721071654.GB7816@wotan.suse.de> References: <1247851850-4298-1-git-send-email-ieidus@redhat.com> <1247851850-4298-2-git-send-email-ieidus@redhat.com> <1247851850-4298-3-git-send-email-ieidus@redhat.com> <1247851850-4298-4-git-send-email-ieidus@redhat.com> <1247851850-4298-5-git-send-email-ieidus@redhat.com> <1247851850-4298-6-git-send-email-ieidus@redhat.com> <4A64B342.8070002@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4A64B342.8070002@redhat.com> Sender: owner-linux-mm@kvack.org To: Rik van Riel Cc: Izik Eidus , akpm@linux-foundation.org, hugh.dickins@tiscali.co.uk, aarcange@redhat.com, chrisw@redhat.com, avi@redhat.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, nickpiggin@yahoo.com.au List-ID: On Mon, Jul 20, 2009 at 02:11:14PM -0400, Rik van Riel wrote: > Izik Eidus wrote: > >From: Hugh Dickins > > > >page_dup_rmap(), used on each mapped page when forking, was originally > >just an inline atomic_inc of mapcount. 2.6.22 added CONFIG_DEBUG_VM > >out-of-line checks to it, which would need to be ever-so-slightly > >complicated to allow for the PageKsm() we're about to define. > > > >But I think these checks never caught anything. And if it's coding > >errors we're worried about, such checks should be in page_remove_rmap() > >too, not just when forking; whereas if it's pagetable corruption we're > >worried about, then they shouldn't be limited to CONFIG_DEBUG_VM. > > Acked-by: Rik van Riel I like debug code like this as it helps comment the code a litte bit too. We've got lots of debug checks in the VM and probably very few of them catch anything useful... I'd kind of like to see it be ever-so-slightly complicated with PageKsm, and even a call to page_check_anon_rmap put into page_remove_rmap (which is a good idea). pagetable corruption/struct page corruption I think is good to check for, but it is fine to have such checks under DEBUG_VM -- we have a couple of orders of magnitude more memory that is not for struct page, so decent coverage of memory corruption kind of wants slab and page debugging too, don't you think? /checks the sky for pigs... -- 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