From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758145Ab2ILCEV (ORCPT ); Tue, 11 Sep 2012 22:04:21 -0400 Received: from mail-qc0-f174.google.com ([209.85.216.174]:65044 "EHLO mail-qc0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753031Ab2ILCEU (ORCPT ); Tue, 11 Sep 2012 22:04:20 -0400 Date: Tue, 11 Sep 2012 19:03:38 -0700 (PDT) From: Hugh Dickins X-X-Sender: hugh@eggly.anvils To: Xiao Guangrong cc: Andrew Morton , Andrea Arcangeli , Michel Lespinasse , David Rientjes , LKML , Linux Memory Management List Subject: Re: [PATCH 09/12] thp: introduce khugepaged_prealloc_page and khugepaged_alloc_page In-Reply-To: <5028E20C.3080607@linux.vnet.ibm.com> Message-ID: References: <5028E12C.70101@linux.vnet.ibm.com> <5028E20C.3080607@linux.vnet.ibm.com> User-Agent: Alpine 2.00 (LSU 1167 2008-08-23) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 13 Aug 2012, Xiao Guangrong wrote: > They are used to abstract the difference between NUMA enabled and NUMA disabled > to make the code more readable > > Signed-off-by: Xiao Guangrong > --- > mm/huge_memory.c | 166 ++++++++++++++++++++++++++++++++---------------------- > 1 files changed, 98 insertions(+), 68 deletions(-) Hmm, that in itself is not necessarily an improvement. I'm a bit sceptical about this patch, thp-introduce-khugepaged_prealloc_page-and-khugepaged_alloc_page.patch in last Thursday's mmotm 2012-09-06-16-46. What brought me to look at it was hitting "BUG at mm/huge_memory.c:1842!" running tmpfs kbuild swapping load (with memcg's memory.limit_in_bytes forcing out to swap), while I happened to have CONFIG_NUMA=y. That's the VM_BUG_ON(*hpage) on entry to khugepaged_alloc_page(). (If I'm honest, I'll admit I have Michel's "interval trees for anon rmap" patches in on top, and so the line number was actually shifted to 1839: but I don't believe his patches were in any way involved here, and indeed I've not yet found a problem with them: they look very good.) I expect the BUG could quite easily be fixed up by making another call to khugepaged_prealloc_page() from somewhere to free up the hpage; but forgive me if I dislike using "prealloc" to free. I do agree with you that the several CONFIG_NUMA ifdefs dotted around mm/huge_memory.c are regrettable, but I'm not at all sure that you're improving the situation with this patch, which gives misleading names to functions and moves the mmap_sem upping out of line. I think you need to revisit it: maybe not go so far (leaving a few CONFIG_NUMAs behind, if they're not too bad), or maybe go further (add a separate function for freeing in the NUMA case, instead of using "prealloc"). I don't know what's best: have a play and see. That's what I was intending to write yesterday. But overnight I was running with this 9/12 backed out (I think 10,11,12 should be independent), and found "BUG at mm/huge_memory.c:1835!" this morning. That's the VM_BUG_ON(*hpage) below #else in collapse_huge_page() when 9/12 is reverted. So maybe 9/12 is just obscuring what was already a BUG, either earlier in your series or elsewhere in mmotm (I've never seen it on 3.6-rc or earlier releases, nor without CONFIG_NUMA). I've not spent any time looking for it, maybe it's obvious - can you spot and fix it? Hugh