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 X-Spam-Level: X-Spam-Status: No, score=-7.2 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 59295C6377A for ; Wed, 21 Jul 2021 22:57:12 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id C0DE861029 for ; Wed, 21 Jul 2021 22:57:11 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org C0DE861029 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 07C7A6B0036; Wed, 21 Jul 2021 18:57:11 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 02D1B6B005D; Wed, 21 Jul 2021 18:57:10 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id E36BB6B006C; Wed, 21 Jul 2021 18:57:10 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0066.hostedemail.com [216.40.44.66]) by kanga.kvack.org (Postfix) with ESMTP id CB24B6B0036 for ; Wed, 21 Jul 2021 18:57:10 -0400 (EDT) Received: from smtpin35.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay01.hostedemail.com (Postfix) with ESMTP id 63A721859DA9E for ; Wed, 21 Jul 2021 22:57:10 +0000 (UTC) X-FDA: 78388107420.35.745B711 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [216.205.24.124]) by imf22.hostedemail.com (Postfix) with ESMTP id EF45D1B3BD for ; Wed, 21 Jul 2021 22:57:09 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1626908229; h=from:from:reply-to:subject:subject: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=nYPuiEUAoQnXgigA8gfgSsVvv5X/dh0PhyM763uouHg=; b=C1ACB10z/fqyjyszcgostSOmOxtCANVZI0GeI4YPMYS0xUw6o32D/qCvdiA3kBvY4s88eV Qq9Y98gHXzstY4i7HxJrDwLTzB/LYvFoYU8kGW1yKth4qqdBlzLV4NZETRJTvd0HSvZn3p 7BlCTYKzawsL+SaPM1ZfYgj8PGkJ2pg= Received: from mail-qt1-f198.google.com (mail-qt1-f198.google.com [209.85.160.198]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-477-GBpK10yGP-imSThUR5RECg-1; Wed, 21 Jul 2021 18:57:02 -0400 X-MC-Unique: GBpK10yGP-imSThUR5RECg-1 Received: by mail-qt1-f198.google.com with SMTP id d7-20020ac85ac70000b029026ae3f4adc9so2366968qtd.13 for ; Wed, 21 Jul 2021 15:57:02 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to; bh=nYPuiEUAoQnXgigA8gfgSsVvv5X/dh0PhyM763uouHg=; b=sDUqTI3cTayyiiDEzdvLoN5YFpukDCkSQMQuwD366RTztZl09BTBQCrsPGQ5GAtN+6 p9De7c2NAn6zqFcjtrrKQLl4idc5ia3SUot90AtQKR0zeA/in/SfIWHFN18adrKDqyjM mzXoW/9k6mtc6Mmn7tcFkikKgDyMLGwHL92/5nCbvDxhd8kgWmHvs3Ilr+Wi6MkJBY6f rNn18Z84RqdcCVSEl4fXZCdYyTLgqH43GRuWqBjO5TeMxG+Y0LSraCKkzYJROrvN7S4Y Zv0xC/q3PLdWwzOrR17rGcQwiYX0BnP513jdOVnUqc3OOk+yklCNEpJsZ7nu5iszhkhH 87Mg== X-Gm-Message-State: AOAM5315dk7+WiFAiamdEtsxxvRyF6yHvJ5kDDD3B3M45M+H4LSm/8CS sbCnfuvAh0BtplaGMOZDzXI/eRluZHSOIWxfQmnsyFgHBUAJ/SZZAXugOAt9oTwHwZrMpw1qAmf 04kX82k+z0Ow= X-Received: by 2002:a37:af45:: with SMTP id y66mr27663733qke.466.1626908222363; Wed, 21 Jul 2021 15:57:02 -0700 (PDT) X-Google-Smtp-Source: ABdhPJx6cIXdiz0a5BMrUZSi6UQdK/SvtgxeRSmtDqXbf0AKl9xdYYxgvfL0PJqb8UPjnwOt+T5xAA== X-Received: by 2002:a37:af45:: with SMTP id y66mr27663703qke.466.1626908222137; Wed, 21 Jul 2021 15:57:02 -0700 (PDT) Received: from t490s (bras-base-toroon474qw-grc-65-184-144-111-238.dsl.bell.ca. [184.144.111.238]) by smtp.gmail.com with ESMTPSA id p22sm9506044qtq.64.2021.07.21.15.57.00 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 21 Jul 2021 15:57:01 -0700 (PDT) Date: Wed, 21 Jul 2021 18:57:00 -0400 From: Peter Xu To: Ivan Teterevkov Cc: David Hildenbrand , Tiberiu Georgescu , "linux-kernel@vger.kernel.org" , "linux-mm@kvack.org" , Axel Rasmussen , Nadav Amit , Jerome Glisse , "Kirill A . Shutemov" , Jason Gunthorpe , Alistair Popple , Andrew Morton , Andrea Arcangeli , Matthew Wilcox , Mike Kravetz , Hugh Dickins , Miaohe Lin , Mike Rapoport , "Carl Waldspurger [C]" , Florian Schmidt , "ovzxemul@gmail.com" Subject: Re: [PATCH v5 24/26] mm/pagemap: Recognize uffd-wp bit for shmem/hugetlbfs Message-ID: References: <20210715201422.211004-1-peterx@redhat.com> <20210715201651.212134-1-peterx@redhat.com> <5c3c84ee-02f6-a2af-13b8-5dcf70676641@redhat.com> MIME-Version: 1.0 In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Authentication-Results: imf22.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=C1ACB10z; spf=none (imf22.hostedemail.com: domain of peterx@redhat.com has no SPF policy when checking 216.205.24.124) smtp.mailfrom=peterx@redhat.com; dmarc=pass (policy=none) header.from=redhat.com X-Stat-Signature: jkjwoyxqpa71w8917e69ha75q9b8qjj5 X-Rspamd-Queue-Id: EF45D1B3BD X-Rspamd-Server: rspam01 X-HE-Tag: 1626908229-980776 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Wed, Jul 21, 2021 at 06:28:03PM -0400, Peter Xu wrote: > Hi, Ivan, > > On Wed, Jul 21, 2021 at 07:54:44PM +0000, Ivan Teterevkov wrote: > > On Wed, Jul 21, 2021 4:20 PM +0000, David Hildenbrand wrote: > > > On 21.07.21 16:38, Ivan Teterevkov wrote: > > > > On Mon, Jul 19, 2021 5:56 PM +0000, Peter Xu wrote: > > > >> I'm also curious what would be the real use to have an accurate > > > >> PM_SWAP accounting. To me current implementation may not provide > > > >> accurate value but should be good enough for most cases. However not > > > >> sure whether it's also true for your use case. > > > > > > > > We want the PM_SWAP bit implemented (for shared memory in the pagemap > > > > interface) to enhance the live migration for some fraction of the > > > > guest VMs that have their pages swapped out to the host swap. Once > > > > those pages are paged in and transferred over network, we then want to > > > > release them with madvise(MADV_PAGEOUT) and preserve the working set > > > > of the guest VMs to reduce the thrashing of the host swap. > > > > > > There are 3 possibilities I think (swap is just another variant of the page cache): > > > > > > 1) The page is not in the page cache, e.g., it resides on disk or in a swap file. > > > pte_none(). > > > 2) The page is in the page cache and is not mapped into the page table. > > > pte_none(). > > > 3) The page is in the page cache and mapped into the page table. > > > !pte_none(). > > > > > > Do I understand correctly that you want to identify 1) and indicate it via > > > PM_SWAP? > > > > Yes, and I also want to outline the context so we're on the same page. > > > > This series introduces the support for userfaultfd-wp for shared memory > > because once a shared page is swapped, its PTE is cleared. Upon retrieval > > from a swap file, there's no way to "recover" the _PAGE_SWP_UFFD_WP flag > > because unlike private memory it's not kept in PTE or elsewhere. > > > > We came across the same issue with PM_SWAP in the pagemap interface, but > > fortunately, there's the place that we could query: the i_pages field of > > the struct address_space (XArray). In https://lkml.org/lkml/2021/7/14/595 > > we do it similarly to what shmem_fault() does when it handles #PF. > > > > Now, in the context of this series, we were exploring whether it makes > > any practical sense to introduce more brand new flags to the special > > PTE to populate the pagemap flags "on the spot" from the given PTE. > > > > However, I can't see how (and why) to achieve that specifically for > > PM_SWAP even with an extra bit: the XArray is precisely what we need for > > the live migration use case. Another flag PM_SOFT_DIRTY suffers the same > > problem as UFFD_WP_SWP_PTE_SPECIAL before this patch series, but we don't > > need it at the moment. > > > > Hope that clarification makes sense? > > Yes it helps, thanks. > > So I can understand now on how that patch comes initially, even if it may not > work for PM_SOFT_DIRTY but it seems working indeed for PM_SWAP. > > However I have a concern that I raised also in the other thread: I think > there'll be an extra and meaningless xa_load() for all the real pte_none()s > that aren't swapped out but just having no page at the back from the very > beginning. That happens much more frequent when the memory being observed by > pagemap is mapped in a huge chunk and sparsely mapped. > > With old code we'll simply skip those ptes, but now I have no idea how much > overhead would a xa_load() brings. > > Btw, I think there's a way to implement such an idea similar to the swap > special uffd-wp pte - when page reclaim of shmem pages, instead of putting a > none pte there maybe we can also have one bit set in the none pte showing that > this pte is swapped out. When the page faulted back we just drop that bit. > > That bit could be also scanned by pagemap code to know that this page was > swapped out. That should be much lighter than xa_load(), and that identifies > immediately from a real none pte just by reading the value. > > Do you think this would work? Btw, I think that's what Tiberiu used to mention, but I think I just changed my mind.. Sorry to have brought such a confusion. So what I think now is: we can set it (instead of zeroing the pte) right at unmapping the pte of page reclaim. Code-wise, that can be a special flag (maybe, TTU_PAGEOUT?) passed over to try_to_unmap() of shrink_page_list() to differenciate from other try_to_unmap()s. I think that bit can also be dropped correctly e.g. when punching a hole in the file, then rmap_walk() can find and drop the marker (I used to suspect uffd-wp bit could get left-overs, but after a second thought here similarly, it seems it won't; as long as hole punching and vma unmapping will always be able to scan those marker ptes, then it seems all right to drop them correctly). But that's my wild thoughts; I could have missed something too. Thanks, -- Peter Xu