From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753483AbbJ0B2X (ORCPT ); Mon, 26 Oct 2015 21:28:23 -0400 Received: from mail-pa0-f54.google.com ([209.85.220.54]:34321 "EHLO mail-pa0-f54.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752442AbbJ0B2W (ORCPT ); Mon, 26 Oct 2015 21:28:22 -0400 Date: Mon, 26 Oct 2015 18:28:13 -0700 (PDT) From: Hugh Dickins X-X-Sender: hugh@eggly.anvils To: Minchan Kim cc: Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Hugh Dickins , Rik van Riel , Mel Gorman , Michal Hocko , Johannes Weiner , "Kirill A. Shutemov" , Vlastimil Babka Subject: Re: [PATCH 3/5] mm: clear PG_dirty to mark page freeable In-Reply-To: <1445236307-895-4-git-send-email-minchan@kernel.org> Message-ID: References: <1445236307-895-1-git-send-email-minchan@kernel.org> <1445236307-895-4-git-send-email-minchan@kernel.org> User-Agent: Alpine 2.11 (LSU 23 2013-08-11) 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, 19 Oct 2015, 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. > > Cc: Hugh Dickins > Signed-off-by: Minchan Kim Acked-by: Hugh Dickins (and patches 1/5 and 2/5 too if you like) > --- > mm/madvise.c | 12 ++++++++++-- > 1 file changed, 10 insertions(+), 2 deletions(-) > > diff --git a/mm/madvise.c b/mm/madvise.c > index fdfb14a78c60..5db546431285 100644 > --- a/mm/madvise.c > +++ b/mm/madvise.c > @@ -312,11 +312,19 @@ 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 > >