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 CF30AC001E0 for ; Wed, 2 Aug 2023 15:36:13 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233790AbjHBPgM (ORCPT ); Wed, 2 Aug 2023 11:36:12 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:56376 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234932AbjHBPfy (ORCPT ); Wed, 2 Aug 2023 11:35:54 -0400 Received: from outbound-smtp07.blacknight.com (outbound-smtp07.blacknight.com [46.22.139.12]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id B5A983C22 for ; Wed, 2 Aug 2023 08:35:20 -0700 (PDT) Received: from mail.blacknight.com (pemlinmail05.blacknight.ie [81.17.254.26]) by outbound-smtp07.blacknight.com (Postfix) with ESMTPS id 8CF841C4097 for ; Wed, 2 Aug 2023 16:16:15 +0100 (IST) Received: (qmail 9455 invoked from network); 2 Aug 2023 15:16:15 -0000 Received: from unknown (HELO techsingularity.net) (mgorman@techsingularity.net@[84.203.20.191]) by 81.17.254.9 with ESMTPSA (AES256-SHA encrypted, authenticated); 2 Aug 2023 15:16:15 -0000 Date: Wed, 2 Aug 2023 16:16:13 +0100 From: Mel Gorman To: David Hildenbrand Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, kvm@vger.kernel.org, linux-kselftest@vger.kernel.org, Andrew Morton , Linus Torvalds , liubo , Peter Xu , Matthew Wilcox , Hugh Dickins , Jason Gunthorpe , John Hubbard , Mel Gorman , Shuah Khan , Paolo Bonzini Subject: Re: [PATCH v2 2/8] smaps: use vm_normal_page_pmd() instead of follow_trans_huge_pmd() Message-ID: <20230802151613.3nyg3xof3gyovlxu@techsingularity.net> References: <20230801124844.278698-1-david@redhat.com> <20230801124844.278698-3-david@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: <20230801124844.278698-3-david@redhat.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Aug 01, 2023 at 02:48:38PM +0200, David Hildenbrand wrote: > We shouldn't be using a GUP-internal helper if it can be avoided. > > Similar to smaps_pte_entry() that uses vm_normal_page(), let's use > vm_normal_page_pmd() that similarly refuses to return the huge zeropage. > > In contrast to follow_trans_huge_pmd(), vm_normal_page_pmd(): > > (1) Will always return the head page, not a tail page of a THP. > > If we'd ever call smaps_account with a tail page while setting "compound > = true", we could be in trouble, because smaps_account() would look at > the memmap of unrelated pages. > > If we're unlucky, that memmap does not exist at all. Before we removed > PG_doublemap, we could have triggered something similar as in > commit 24d7275ce279 ("fs/proc: task_mmu.c: don't read mapcount for > migration entry"). > > This can theoretically happen ever since commit ff9f47f6f00c ("mm: proc: > smaps_rollup: do not stall write attempts on mmap_lock"): > > (a) We're in show_smaps_rollup() and processed a VMA > (b) We release the mmap lock in show_smaps_rollup() because it is > contended > (c) We merged that VMA with another VMA > (d) We collapsed a THP in that merged VMA at that position > > If the end address of the original VMA falls into the middle of a THP > area, we would call smap_gather_stats() with a start address that falls > into a PMD-mapped THP. It's probably very rare to trigger when not > really forced. > > (2) Will succeed on a is_pci_p2pdma_page(), like vm_normal_page() > > Treat such PMDs here just like smaps_pte_entry() would treat such PTEs. > If such pages would be anonymous, we most certainly would want to > account them. > > (3) Will skip over pmd_devmap(), like vm_normal_page() for pte_devmap() > > As noted in vm_normal_page(), that is only for handling legacy ZONE_DEVICE > pages. So just like smaps_pte_entry(), we'll now also ignore such PMD > entries. > > Especially, follow_pmd_mask() never ends up calling > follow_trans_huge_pmd() on pmd_devmap(). Instead it calls > follow_devmap_pmd() -- which will fail if neither FOLL_GET nor FOLL_PIN > is set. > > So skipping pmd_devmap() pages seems to be the right thing to do. > > (4) Will properly handle VM_MIXEDMAP/VM_PFNMAP, like vm_normal_page() > > We won't be returning a memmap that should be ignored by core-mm, or > worse, a memmap that does not even exist. Note that while > walk_page_range() will skip VM_PFNMAP mappings, walk_page_vma() won't. > > Most probably this case doesn't currently really happen on the PMD level, > otherwise we'd already be able to trigger kernel crashes when reading > smaps / smaps_rollup. > > So most probably only (1) is relevant in practice as of now, but could only > cause trouble in extreme corner cases. > > Fixes: ff9f47f6f00c ("mm: proc: smaps_rollup: do not stall write attempts on mmap_lock") > Signed-off-by: David Hildenbrand Maybe move the follow_trans_huge_pmd() declaration from linux/huge_mm.h to mm/internal.h to discourage future mistakes? Otherwise Acked-by: Mel Gorman -- Mel Gorman SUSE Labs