From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <4637EC95.2010501@yahoo.com.au> Date: Wed, 02 May 2007 11:42:45 +1000 From: Nick Piggin MIME-Version: 1.0 Subject: Re: 2.6.22 -mm merge plans: mm-more-rmap-checking References: <20070430162007.ad46e153.akpm@linux-foundation.org> In-Reply-To: Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org Return-Path: To: Hugh Dickins Cc: Andrew Morton , linux-kernel@vger.kernel.org, linux-mm@kvack.org List-ID: Hugh Dickins wrote: > On Mon, 30 Apr 2007, Andrew Morton wrote: > >>... >> mm-more-rmap-checking.patch >>... >> >>Misc MM things. Will merge. > > > Would Nick mind very much if I ask you to drop this one? > You did CC me ages ago, but I've only just run across it. > It's a small matter, but I'd prefer it dropped for now. I guess I would prefer it to go under CONFIG_DEBUG_VM. Speaking of which, it would be nice to be able to turn that on unconditionally in -rc1. Although I may have put a few too many things under it, so it might slow down too much... >>>Re-introduce rmap verification patches that Hugh removed when he removed >>>PG_map_lock. PG_map_lock actually isn't needed to synchronise access to >>>anonymous pages, because PG_locked and PTL together already do. >>> >>>These checks were important in discovering and fixing a rare rmap corruption >>>in SLES9. > > > It introduces some silly checks which were never in mainline, > nor so far as I can tell in SLES9: I'm thinking of those > + BUG_ON(address < vma->vm_start || address >= vma->vm_end); Yes, but IIRC I put that in because there was another check in SLES9 that I actually couldn't put in, but used this one instead because it also caught the bug we saw. > There are few callsites for these rmap functions, I don't think > they need to be checking their arguments in that way. > > It also changes the inline page_dup_rmap (a single atomic increment) > into a bugchecking out-of-line function: do we really want to slow > down fork in that way, for 2.6.22 to fix a rare corruption in SLES9? This was actually a rare corruption that is also in 2.6.21, and as few rmap callsites as we have, it was never noticed until the SLES9 bug check was triggered. > What I really like about the patch is Nick's observation that my > /* else checking page index and mapping is racy */ > is no longer true: a change we made to the do_swap_page sequence > some while ago has indeed cured that raciness, and I'm happy to > reintroduce the check on mapping and index in page_add_anon_rmap, > and his BUG_ON(!PageLocked(page)) there (despite BUG_ONs falling > out of fashion very recently). Hmm, I didn't notice the do_swap_page change, rather just derived its safety by looking at the current state of the code (which I guess must have been post-do_swap_page change)... Do you have a pointer to the patch, for my interest? -- SUSE Labs, Novell Inc. -- 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