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=-1.0 required=3.0 tests=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 3F8B5C433E0 for ; Wed, 24 Jun 2020 14:22:13 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id F0E3E2072E for ; Wed, 24 Jun 2020 14:22:12 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org F0E3E2072E Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=chris-wilson.co.uk Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 633DE6B0006; Wed, 24 Jun 2020 10:22:12 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 5E3EC6B0007; Wed, 24 Jun 2020 10:22:12 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 4FADC6B0008; Wed, 24 Jun 2020 10:22:12 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0142.hostedemail.com [216.40.44.142]) by kanga.kvack.org (Postfix) with ESMTP id 3790F6B0006 for ; Wed, 24 Jun 2020 10:22:12 -0400 (EDT) Received: from smtpin08.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay04.hostedemail.com (Postfix) with ESMTP id EA9291390E for ; Wed, 24 Jun 2020 14:22:11 +0000 (UTC) X-FDA: 76964320062.08.toes39_3617ffc26e44 Received: from filter.hostedemail.com (10.5.16.251.rfc1918.com [10.5.16.251]) by smtpin08.hostedemail.com (Postfix) with ESMTP id 784301802718C for ; Wed, 24 Jun 2020 14:22:00 +0000 (UTC) X-HE-Tag: toes39_3617ffc26e44 X-Filterd-Recvd-Size: 3718 Received: from fireflyinternet.com (mail.fireflyinternet.com [109.228.58.192]) by imf41.hostedemail.com (Postfix) with ESMTP for ; Wed, 24 Jun 2020 14:21:59 +0000 (UTC) X-Default-Received-SPF: pass (skip=forwardok (res=PASS)) x-ip-name=78.156.65.138; Received: from localhost (unverified [78.156.65.138]) by fireflyinternet.com (Firefly Internet (M1)) with ESMTP (TLS) id 21603718-1500050 for multiple; Wed, 24 Jun 2020 15:21:54 +0100 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable In-Reply-To: <20200624141604.GH6578@ziepe.ca> References: <20200624080248.3701-1-chris@chris-wilson.co.uk> <20200624121053.GD6578@ziepe.ca> <159300126338.4527.3968787379471939056@build.alporthouse.com> <20200624123910.GA3178169@ziepe.ca> <159300796224.4527.2014771396582759689@build.alporthouse.com> <20200624141604.GH6578@ziepe.ca> From: Chris Wilson Subject: Re: [PATCH 1/2] mm/mmu_notifier: Mark up direct reclaim paths with MAYFAIL To: Jason Gunthorpe Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, intel-gfx@lists.freedesktop.org, Andrew Morton Message-ID: <159300850942.4527.8335506003268197914@build.alporthouse.com> User-Agent: alot/0.8.1 Date: Wed, 24 Jun 2020 15:21:49 +0100 X-Rspamd-Queue-Id: 784301802718C X-Spamd-Result: default: False [0.00 / 100.00] X-Rspamd-Server: rspam03 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: Quoting Jason Gunthorpe (2020-06-24 15:16:04) > On Wed, Jun 24, 2020 at 03:12:42PM +0100, Chris Wilson wrote: > > Quoting Jason Gunthorpe (2020-06-24 13:39:10) > > > On Wed, Jun 24, 2020 at 01:21:03PM +0100, Chris Wilson wrote: > > > > Quoting Jason Gunthorpe (2020-06-24 13:10:53) > > > > > On Wed, Jun 24, 2020 at 09:02:47AM +0100, Chris Wilson wrote: > > > > > > When direct reclaim enters the shrinker and tries to reclaim pa= ges, it > > > > > > has to opportunitically unmap them [try_to_unmap_one]. For dire= ct > > > > > > reclaim, the calling context is unknown and may include attempt= s to > > > > > > unmap one page of a dma object while attempting to allocate mor= e pages > > > > > > for that object. Pass the information along that we are inside = an > > > > > > opportunistic unmap that can allow that page to remain referenc= ed and > > > > > > mapped, and let the callback opt in to avoiding a recursive wai= t. > > > > >=20 > > > > > i915 should already not be holding locks shared with the notifiers > > > > > across allocations that can trigger reclaim. This is already requ= ired > > > > > to use notifiers correctly anyhow - why do we need something in t= he > > > > > notifiers? > > > >=20 > > > > for (n =3D 0; n < num_pages; n++) > > > > pin_user_page() > > > >=20 > > > > may call try_to_unmap_page from the lru shrinker for [0, n-1]. > > >=20 > > > Yes, of course you can't hold any locks that intersect with notifiers > > > across pin_user_page()/get_user_page() > >=20 > > What lock though? It's just the page refcount, shrinker asks us to drop > > it [via mmu], we reply we would like to keep using that page as freeing > > it for the current allocation is "robbing Peter to pay Paul". >=20 > Maybe I'm unclear what this series is actually trying to fix?=20 >=20 > You said "avoiding a recursive wait" which sounds like some locking > deadlock to me. It's the shrinker being called while we are allocating for/on behalf of the object. As we are actively using the object, we don't want to free it -- the partial object allocation being the clearest, if the object consists of 2 pages, trying to free page 0 in order to allocate page 1 has to fail (and the shrinker should find another candidate to reclaim, or fail the allocation). -Chris