From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1030609AbbJ3MzK (ORCPT ); Fri, 30 Oct 2015 08:55:10 -0400 Received: from mail-wm0-f53.google.com ([74.125.82.53]:33869 "EHLO mail-wm0-f53.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S964969AbbJ3MzH (ORCPT ); Fri, 30 Oct 2015 08:55:07 -0400 Date: Fri, 30 Oct 2015 13:55:04 +0100 From: Michal Hocko To: Minchan Kim Cc: Andrew Morton , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Michael Kerrisk , linux-api@vger.kernel.org, Hugh Dickins , Johannes Weiner , zhangyanfei@cn.fujitsu.com, Rik van Riel , Mel Gorman , KOSAKI Motohiro , Jason Evans , Daniel Micay , "Kirill A. Shutemov" , yalin.wang2010@gmail.com, Shaohua Li Subject: Re: [PATCH 7/8] mm: clear PG_dirty to mark page freeable Message-ID: <20151030125504.GC23627@dhcp22.suse.cz> References: <1446188504-28023-1-git-send-email-minchan@kernel.org> <1446188504-28023-8-git-send-email-minchan@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1446188504-28023-8-git-send-email-minchan@kernel.org> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri 30-10-15 16:01:43, Minchan Kim wrote: > Basically, MADV_FREE relies on dirty bit in page table entry to decide > whether VM allows to discard the page or not. IOW, if page table entry > includes marked dirty bit, VM shouldn't discard the page. > > However, as a example, if swap-in by read fault happens, page table entry > doesn't have dirty bit so MADV_FREE could discard the page wrongly. > > For avoiding the problem, MADV_FREE did more checks with PageDirty > and PageSwapCache. It worked out because swapped-in page lives on > swap cache and since it is evicted from the swap cache, the page has > PG_dirty flag. So both page flags check effectively prevent > wrong discarding by MADV_FREE. > > However, a problem in above logic is that swapped-in page has > PG_dirty still after they are removed from swap cache so VM cannot > consider the page as freeable any more even if madvise_free is > called in future. > > Look at below example for detail. > > ptr = malloc(); > memset(ptr); > .. > .. > .. heavy memory pressure so all of pages are swapped out > .. > .. > var = *ptr; -> a page swapped-in and could be removed from > swapcache. Then, page table doesn't mark > dirty bit and page descriptor includes PG_dirty > .. > .. > madvise_free(ptr); -> It doesn't clear PG_dirty of the page. > .. > .. > .. > .. heavy memory pressure again. > .. In this time, VM cannot discard the page because the page > .. has *PG_dirty* > > To solve the problem, this patch clears PG_dirty if only the page is owned > exclusively by current process when madvise is called because PG_dirty > represents ptes's dirtiness in several processes so we could clear it only > if we own it exclusively. > > Acked-by: Hugh Dickins > Signed-off-by: Minchan Kim Acked-by: Michal Hocko > --- > mm/madvise.c | 12 ++++++++++-- > 1 file changed, 10 insertions(+), 2 deletions(-) > > diff --git a/mm/madvise.c b/mm/madvise.c > index 9ee9df8c768d..fc24104d6b3a 100644 > --- a/mm/madvise.c > +++ b/mm/madvise.c > @@ -303,11 +303,19 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr, > if (!page) > continue; > > - if (PageSwapCache(page)) { > + if (PageSwapCache(page) || PageDirty(page)) { > if (!trylock_page(page)) > continue; > + /* > + * If page is shared with others, we couldn't clear > + * PG_dirty of the page. > + */ > + if (page_count(page) != 1 + !!PageSwapCache(page)) { > + unlock_page(page); > + continue; > + } > > - if (!try_to_free_swap(page)) { > + if (PageSwapCache(page) && !try_to_free_swap(page)) { > unlock_page(page); > continue; > } > -- > 1.9.1 -- Michal Hocko SUSE Labs