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=-5.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=unavailable 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 97983C32753 for ; Wed, 14 Aug 2019 15:49:33 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 6E25F2133F for ; Wed, 14 Aug 2019 15:49:33 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727650AbfHNPt3 (ORCPT ); Wed, 14 Aug 2019 11:49:29 -0400 Received: from mx1.redhat.com ([209.132.183.28]:36156 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726166AbfHNPt2 (ORCPT ); Wed, 14 Aug 2019 11:49:28 -0400 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 760DE30A7BB3; Wed, 14 Aug 2019 15:49:27 +0000 (UTC) Received: from [10.18.17.163] (dhcp-17-163.bos.redhat.com [10.18.17.163]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 5C3B9100EBA3; Wed, 14 Aug 2019 15:49:05 +0000 (UTC) Subject: Re: [RFC][Patch v12 1/2] mm: page_reporting: core infrastructure To: Alexander Duyck Cc: kvm list , LKML , linux-mm , virtio-dev@lists.oasis-open.org, Paolo Bonzini , lcapitulino@redhat.com, pagupta@redhat.com, wei.w.wang@intel.com, Yang Zhang , Rik van Riel , David Hildenbrand , "Michael S. Tsirkin" , dodgen@google.com, Konrad Rzeszutek Wilk , dhildenb@redhat.com, Andrea Arcangeli , john.starks@microsoft.com, Dave Hansen , Michal Hocko , cohuck@redhat.com References: <20190812131235.27244-1-nitesh@redhat.com> <20190812131235.27244-2-nitesh@redhat.com> From: Nitesh Narayan Lal Openpgp: preference=signencrypt Autocrypt: addr=nitesh@redhat.com; prefer-encrypt=mutual; keydata= mQINBFl4pQoBEADT/nXR2JOfsCjDgYmE2qonSGjkM1g8S6p9UWD+bf7YEAYYYzZsLtbilFTe z4nL4AV6VJmC7dBIlTi3Mj2eymD/2dkKP6UXlliWkq67feVg1KG+4UIp89lFW7v5Y8Muw3Fm uQbFvxyhN8n3tmhRe+ScWsndSBDxYOZgkbCSIfNPdZrHcnOLfA7xMJZeRCjqUpwhIjxQdFA7 n0s0KZ2cHIsemtBM8b2WXSQG9CjqAJHVkDhrBWKThDRF7k80oiJdEQlTEiVhaEDURXq+2XmG jpCnvRQDb28EJSsQlNEAzwzHMeplddfB0vCg9fRk/kOBMDBtGsTvNT9OYUZD+7jaf0gvBvBB lbKmmMMX7uJB+ejY7bnw6ePNrVPErWyfHzR5WYrIFUtgoR3LigKnw5apzc7UIV9G8uiIcZEn C+QJCK43jgnkPcSmwVPztcrkbC84g1K5v2Dxh9amXKLBA1/i+CAY8JWMTepsFohIFMXNLj+B RJoOcR4HGYXZ6CAJa3Glu3mCmYqHTOKwezJTAvmsCLd3W7WxOGF8BbBjVaPjcZfavOvkin0u DaFvhAmrzN6lL0msY17JCZo046z8oAqkyvEflFbC0S1R/POzehKrzQ1RFRD3/YzzlhmIowkM BpTqNBeHEzQAlIhQuyu1ugmQtfsYYq6FPmWMRfFPes/4JUU/PQARAQABtCVOaXRlc2ggTmFy YXlhbiBMYWwgPG5pbGFsQHJlZGhhdC5jb20+iQI9BBMBCAAnBQJZeKUKAhsjBQkJZgGABQsJ CAcCBhUICQoLAgQWAgMBAh4BAheAAAoJEKOGQNwGMqM56lEP/A2KMs/pu0URcVk/kqVwcBhU SnvB8DP3lDWDnmVrAkFEOnPX7GTbactQ41wF/xwjwmEmTzLrMRZpkqz2y9mV0hWHjqoXbOCS 6RwK3ri5e2ThIPoGxFLt6TrMHgCRwm8YuOSJ97o+uohCTN8pmQ86KMUrDNwMqRkeTRW9wWIQ EdDqW44VwelnyPwcmWHBNNb1Kd8j3xKlHtnS45vc6WuoKxYRBTQOwI/5uFpDZtZ1a5kq9Ak/ MOPDDZpd84rqd+IvgMw5z4a5QlkvOTpScD21G3gjmtTEtyfahltyDK/5i8IaQC3YiXJCrqxE r7/4JMZeOYiKpE9iZMtS90t4wBgbVTqAGH1nE/ifZVAUcCtycD0f3egX9CHe45Ad4fsF3edQ ESa5tZAogiA4Hc/yQpnnf43a3aQ67XPOJXxS0Qptzu4vfF9h7kTKYWSrVesOU3QKYbjEAf95 NewF9FhAlYqYrwIwnuAZ8TdXVDYt7Z3z506//sf6zoRwYIDA8RDqFGRuPMXUsoUnf/KKPrtR ceLcSUP/JCNiYbf1/QtW8S6Ca/4qJFXQHp0knqJPGmwuFHsarSdpvZQ9qpxD3FnuPyo64S2N Dfq8TAeifNp2pAmPY2PAHQ3nOmKgMG8Gn5QiORvMUGzSz8Lo31LW58NdBKbh6bci5+t/HE0H pnyVf5xhNC/FuQINBFl4pQoBEACr+MgxWHUP76oNNYjRiNDhaIVtnPRqxiZ9v4H5FPxJy9UD Bqr54rifr1E+K+yYNPt/Po43vVL2cAyfyI/LVLlhiY4yH6T1n+Di/hSkkviCaf13gczuvgz4 KVYLwojU8+naJUsiCJw01MjO3pg9GQ+47HgsnRjCdNmmHiUQqksMIfd8k3reO9SUNlEmDDNB XuSzkHjE5y/R/6p8uXaVpiKPfHoULjNRWaFc3d2JGmxJpBdpYnajoz61m7XJlgwl/B5Ql/6B dHGaX3VHxOZsfRfugwYF9CkrPbyO5PK7yJ5vaiWre7aQ9bmCtXAomvF1q3/qRwZp77k6i9R3 tWfXjZDOQokw0u6d6DYJ0Vkfcwheg2i/Mf/epQl7Pf846G3PgSnyVK6cRwerBl5a68w7xqVU 4KgAh0DePjtDcbcXsKRT9D63cfyfrNE+ea4i0SVik6+N4nAj1HbzWHTk2KIxTsJXypibOKFX 2VykltxutR1sUfZBYMkfU4PogE7NjVEU7KtuCOSAkYzIWrZNEQrxYkxHLJsWruhSYNRsqVBy KvY6JAsq/i5yhVd5JKKU8wIOgSwC9P6mXYRgwPyfg15GZpnw+Fpey4bCDkT5fMOaCcS+vSU1 UaFmC4Ogzpe2BW2DOaPU5Ik99zUFNn6cRmOOXArrryjFlLT5oSOe4IposgWzdwARAQABiQIl BBgBCAAPBQJZeKUKAhsMBQkJZgGAAAoJEKOGQNwGMqM5ELoP/jj9d9gF1Al4+9bngUlYohYu 0sxyZo9IZ7Yb7cHuJzOMqfgoP4tydP4QCuyd9Q2OHHL5AL4VFNb8SvqAxxYSPuDJTI3JZwI7 d8JTPKwpulMSUaJE8ZH9n8A/+sdC3CAD4QafVBcCcbFe1jifHmQRdDrvHV9Es14QVAOTZhnJ vweENyHEIxkpLsyUUDuVypIo6y/Cws+EBCWt27BJi9GH/EOTB0wb+2ghCs/i3h8a+bi+bS7L FCCm/AxIqxRurh2UySn0P/2+2eZvneJ1/uTgfxnjeSlwQJ1BWzMAdAHQO1/lnbyZgEZEtUZJ x9d9ASekTtJjBMKJXAw7GbB2dAA/QmbA+Q+Xuamzm/1imigz6L6sOt2n/X/SSc33w8RJUyor SvAIoG/zU2Y76pKTgbpQqMDmkmNYFMLcAukpvC4ki3Sf086TdMgkjqtnpTkEElMSFJC8npXv 3QnGGOIfFug/qs8z03DLPBz9VYS26jiiN7QIJVpeeEdN/LKnaz5LO+h5kNAyj44qdF2T2AiF HxnZnxO5JNP5uISQH3FjxxGxJkdJ8jKzZV7aT37sC+Rp0o3KNc+GXTR+GSVq87Xfuhx0LRST NK9ZhT0+qkiN7npFLtNtbzwqaqceq3XhafmCiw8xrtzCnlB/C4SiBr/93Ip4kihXJ0EuHSLn VujM7c/b4pps Organization: Red Hat Inc, Message-ID: <6d5b57ca-41ff-5c54-ab20-2b1631a6ce29@redhat.com> Date: Wed, 14 Aug 2019 11:49:04 -0400 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Content-Language: en-US X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.47]); Wed, 14 Aug 2019 15:49:28 +0000 (UTC) Sender: kvm-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On 8/12/19 2:47 PM, Alexander Duyck wrote: > On Mon, Aug 12, 2019 at 6:13 AM Nitesh Narayan Lal wrote: >> This patch introduces the core infrastructure for free page reporting in >> virtual environments. It enables the kernel to track the free pages which >> can be reported to its hypervisor so that the hypervisor could >> free and reuse that memory as per its requirement. >> >> While the pages are getting processed in the hypervisor (e.g., >> via MADV_DONTNEED), the guest must not use them, otherwise, data loss >> would be possible. To avoid such a situation, these pages are >> temporarily removed from the buddy. The amount of pages removed >> temporarily from the buddy is governed by the backend(virtio-balloon >> in our case). >> >> To efficiently identify free pages that can to be reported to the >> hypervisor, bitmaps in a coarse granularity are used. Only fairly big >> chunks are reported to the hypervisor - especially, to not break up THP >> in the hypervisor - "MAX_ORDER - 2" on x86, and to save space. The bits >> in the bitmap are an indication whether a page *might* be free, not a >> guarantee. A new hook after buddy merging sets the bits. >> >> Bitmaps are stored per zone, protected by the zone lock. A workqueue >> asynchronously processes the bitmaps, trying to isolate and report pages >> that are still free. The backend (virtio-balloon) is responsible for >> reporting these batched pages to the host synchronously. Once reporting/ >> freeing is complete, isolated pages are returned back to the buddy. >> >> Signed-off-by: Nitesh Narayan Lal > [...] >> +} >> + >> +/** >> + * __page_reporting_enqueue - tracks the freed page in the respective zone's >> + * bitmap and enqueues a new page reporting job to the workqueue if possible. >> + */ >> +void __page_reporting_enqueue(struct page *page) >> +{ >> + struct page_reporting_config *phconf; >> + struct zone *zone; >> + >> + rcu_read_lock(); >> + /* >> + * We should not process this page if either page reporting is not >> + * yet completely enabled or it has been disabled by the backend. >> + */ >> + phconf = rcu_dereference(page_reporting_conf); >> + if (!phconf) >> + return; >> + >> + zone = page_zone(page); >> + bitmap_set_bit(page, zone); >> + >> + /* >> + * We should not enqueue a job if a previously enqueued reporting work >> + * is in progress or we don't have enough free pages in the zone. >> + */ >> + if (atomic_read(&zone->free_pages) >= phconf->max_pages && >> + !atomic_cmpxchg(&phconf->refcnt, 0, 1)) > This doesn't make any sense to me. Why are you only incrementing the > refcount if it is zero? Combining this with the assignment above, this > isn't really a refcnt. It is just an oversized bitflag. The intent for having an extra variable was to ensure that at a time only one reporting job is enqueued. I do agree that for that purpose I really don't need a reference counter and I should have used something like bool 'page_hinting_active'. But with bool, I think there could be a possible chance of race. Maybe I should rename this variable and keep it as atomic. Any thoughts? > > Also I am pretty sure this results in the opportunity to miss pages > because there is nothing to prevent you from possibly missing a ton of > pages you could hint on if a large number of pages are pushed out all > at once and then the system goes idle in terms of memory allocation > and freeing. I was looking at how you are enqueuing/processing reporting jobs for each zone. I am wondering if I should also consider something on similar lines as having that I might be able to address the concern which you have raised above. But it would also mean that I have to add an additional flag in the zone_flags. :) > [...] > >> +EXPORT_SYMBOL_GPL(page_reporting_enable); >> -- >> 2.21.0 >> -- Thanks Nitesh