From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 0A0F7EE7FF3 for ; Fri, 8 Sep 2023 13:07:53 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S238549AbjIHNHy (ORCPT ); Fri, 8 Sep 2023 09:07:54 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:44890 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229699AbjIHNHx (ORCPT ); Fri, 8 Sep 2023 09:07:53 -0400 Received: from smtp-out1.suse.de (smtp-out1.suse.de [IPv6:2001:67c:2178:6::1c]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 50B6919B5; Fri, 8 Sep 2023 06:07:47 -0700 (PDT) Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by smtp-out1.suse.de (Postfix) with ESMTPS id F31FC21C23; Fri, 8 Sep 2023 13:07:45 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1694178466; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=vPgKK4KO37ns8G+nPIZgZtzOD5MvE8KIepdykhSGMjs=; b=KATbYwB+f8mgA+lX/GSP3aQyY9jKpzeIDpll3DF8Zm1liaipzDofX9jMRj0shiEtCRfr4H XsvbOV3yVGMAUom+PPocJ6MXrLAr/07L50+R+sViBh+XL0e6qU8PzCiMpTUuVl1wOjYlh+ Ye6yxRPOktnYS4yu8z4Dfw6ogrFxU1A= Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by imap2.suse-dmz.suse.de (Postfix) with ESMTPS id CDD29131FD; Fri, 8 Sep 2023 13:07:45 +0000 (UTC) Received: from dovecot-director2.suse.de ([192.168.254.65]) by imap2.suse-dmz.suse.de with ESMTPSA id iclrLqEc+2T/TgAAMHmgww (envelope-from ); Fri, 08 Sep 2023 13:07:45 +0000 Date: Fri, 8 Sep 2023 15:07:45 +0200 From: Michal Hocko To: "Joel Fernandes (Google)" Cc: linux-kernel@vger.kernel.org, Lorenzo Stoakes , Linus Torvalds , linux-kselftest@vger.kernel.org, linux-mm@kvack.org, Shuah Khan , Vlastimil Babka , Kirill A Shutemov , "Liam R. Howlett" , "Paul E. McKenney" , Suren Baghdasaryan , Kalesh Singh , Lokesh Gidra Subject: Re: [PATCH v6 1/7] mm/mremap: Optimize the start addresses in move_page_tables() Message-ID: References: <20230903151328.2981432-1-joel@joelfernandes.org> <20230903151328.2981432-2-joel@joelfernandes.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230903151328.2981432-2-joel@joelfernandes.org> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Sorry for being silent most of the time in this patch series and thanks for pushing it forward. On Sun 03-09-23 15:13:22, Joel Fernandes wrote: > Recently, we see reports [1] of a warning that triggers due to > move_page_tables() doing a downward and overlapping move on a > mutually-aligned offset within a PMD. By mutual alignment, I > mean the source and destination addresses of the mremap are at > the same offset within a PMD. > > This mutual alignment along with the fact that the move is downward is > sufficient to cause a warning related to having an allocated PMD that > does not have PTEs in it. > > This warning will only trigger when there is mutual alignment in the > move operation. A solution, as suggested by Linus Torvalds [2], is to > initiate the copy process at the PMD level whenever such alignment is > present. Implementing this approach will not only prevent the warning > from being triggered, but it will also optimize the operation as this > method should enhance the speed of the copy process whenever there's a > possibility to start copying at the PMD level. > > Some more points: > a. The optimization can be done only when both the source and > destination of the mremap do not have anything mapped below it up to a > PMD boundary. I add support to detect that. > > b. #1 is not a problem for the call to move_page_tables() from exec.c as > nothing is expected to be mapped below the source. However, for > non-overlapping mutually aligned moves as triggered by mremap(2), I > added support for checking such cases. > > c. I currently only optimize for PMD moves, in the future I/we can build > on this work and do PUD moves as well if there is a need for this. But I > want to take it one step at a time. > > d. We need to be careful about mremap of ranges within the VMA itself. > For this purpose, I added checks to determine if the address after > alignment falls within its VMA itself. > > [1] https://lore.kernel.org/all/ZB2GTBD%2FLWTrkOiO@dhcp22.suse.cz/ > [2] https://lore.kernel.org/all/CAHk-=whd7msp8reJPfeGNyt0LiySMT0egExx3TVZSX3Ok6X=9g@mail.gmail.com/ > > Reviewed-by: Lorenzo Stoakes > Suggested-by: Linus Torvalds > Signed-off-by: Joel Fernandes (Google) The patch looks good to me. Acked-by: Michal Hocko > --- > mm/mremap.c | 62 +++++++++++++++++++++++++++++++++++++++++++++++++++++ > 1 file changed, 62 insertions(+) > > diff --git a/mm/mremap.c b/mm/mremap.c > index 11e06e4ab33b..1011326b7b80 100644 > --- a/mm/mremap.c > +++ b/mm/mremap.c > @@ -489,6 +489,53 @@ static bool move_pgt_entry(enum pgt_entry entry, struct vm_area_struct *vma, > return moved; > } > > +/* > + * A helper to check if a previous mapping exists. Required for > + * move_page_tables() and realign_addr() to determine if a previous mapping > + * exists before we can do realignment optimizations. > + */ > +static bool can_align_down(struct vm_area_struct *vma, unsigned long addr_to_align, > + unsigned long mask) > +{ > + unsigned long addr_masked = addr_to_align & mask; > + > + /* > + * If @addr_to_align of either source or destination is not the beginning > + * of the corresponding VMA, we can't align down or we will destroy part > + * of the current mapping. > + */ > + if (vma->vm_start != addr_to_align) > + return false; > + > + /* > + * Make sure the realignment doesn't cause the address to fall on an > + * existing mapping. > + */ > + return find_vma_intersection(vma->vm_mm, addr_masked, vma->vm_start) == NULL; > +} > + > +/* Opportunistically realign to specified boundary for faster copy. */ > +static void try_realign_addr(unsigned long *old_addr, struct vm_area_struct *old_vma, > + unsigned long *new_addr, struct vm_area_struct *new_vma, > + unsigned long mask) > +{ > + /* Skip if the addresses are already aligned. */ > + if ((*old_addr & ~mask) == 0) > + return; > + > + /* Only realign if the new and old addresses are mutually aligned. */ > + if ((*old_addr & ~mask) != (*new_addr & ~mask)) > + return; > + > + /* Ensure realignment doesn't cause overlap with existing mappings. */ > + if (!can_align_down(old_vma, *old_addr, mask) || > + !can_align_down(new_vma, *new_addr, mask)) > + return; > + > + *old_addr = *old_addr & mask; > + *new_addr = *new_addr & mask; > +} > + > unsigned long move_page_tables(struct vm_area_struct *vma, > unsigned long old_addr, struct vm_area_struct *new_vma, > unsigned long new_addr, unsigned long len, > @@ -508,6 +555,14 @@ unsigned long move_page_tables(struct vm_area_struct *vma, > return move_hugetlb_page_tables(vma, new_vma, old_addr, > new_addr, len); > > + /* > + * If possible, realign addresses to PMD boundary for faster copy. > + * Only realign if the mremap copying hits a PMD boundary. > + */ > + if ((vma != new_vma) > + && (len >= PMD_SIZE - (old_addr & ~PMD_MASK))) > + try_realign_addr(&old_addr, vma, &new_addr, new_vma, PMD_MASK); > + > flush_cache_range(vma, old_addr, old_end); > mmu_notifier_range_init(&range, MMU_NOTIFY_UNMAP, 0, vma->vm_mm, > old_addr, old_end); > @@ -577,6 +632,13 @@ unsigned long move_page_tables(struct vm_area_struct *vma, > > mmu_notifier_invalidate_range_end(&range); > > + /* > + * Prevent negative return values when {old,new}_addr was realigned > + * but we broke out of the above loop for the first PMD itself. > + */ > + if (len + old_addr < old_end) > + return 0; > + > return len + old_addr - old_end; /* how much done */ > } > > -- > 2.42.0.283.g2d96d420d3-goog -- Michal Hocko SUSE Labs