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=-0.8 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 2EDEDC76192 for ; Wed, 17 Jul 2019 13:17:36 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 115A221743 for ; Wed, 17 Jul 2019 13:17:36 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727092AbfGQNRf convert rfc822-to-8bit (ORCPT ); Wed, 17 Jul 2019 09:17:35 -0400 Received: from mail.fireflyinternet.com ([109.228.58.192]:49592 "EHLO fireflyinternet.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1727061AbfGQNRf (ORCPT ); Wed, 17 Jul 2019 09:17:35 -0400 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 17371140-1500050 for multiple; Wed, 17 Jul 2019 14:17:27 +0100 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 8BIT To: Tvrtko Ursulin , intel-gfx@lists.freedesktop.org From: Chris Wilson In-Reply-To: Cc: stable@vger.kernel.org References: <20190716124931.5870-1-chris@chris-wilson.co.uk> <156329142200.9436.8651620549785965913@skylake-alporthouse-com> Message-ID: <156336944635.4375.7269371478914847980@skylake-alporthouse-com> User-Agent: alot/0.6 Subject: Re: [Intel-gfx] [PATCH 1/5] drm/i915/userptr: Beware recursive lock_page() Date: Wed, 17 Jul 2019 14:17:26 +0100 Sender: stable-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: stable@vger.kernel.org Quoting Tvrtko Ursulin (2019-07-17 14:09:00) > > On 16/07/2019 16:37, Chris Wilson wrote: > > Quoting Tvrtko Ursulin (2019-07-16 16:25:22) > >> > >> On 16/07/2019 13:49, Chris Wilson wrote: > >>> Following a try_to_unmap() we may want to remove the userptr and so call > >>> put_pages(). However, try_to_unmap() acquires the page lock and so we > >>> must avoid recursively locking the pages ourselves -- which means that > >>> we cannot safely acquire the lock around set_page_dirty(). Since we > >>> can't be sure of the lock, we have to risk skip dirtying the page, or > >>> else risk calling set_page_dirty() without a lock and so risk fs > >>> corruption. > >> > >> So if trylock randomly fail we get data corruption in whatever data set > >> application is working on, which is what the original patch was trying > >> to avoid? Are we able to detect the backing store type so at least we > >> don't risk skipping set_page_dirty with anonymous/shmemfs? > > > > page->mapping??? > > Would page->mapping work? What is it telling us? It basically tells us if there is a fs around; anything that is the most basic of malloc (even tmpfs/shmemfs has page->mapping). > > We still have the issue that if there is a mapping we should be taking > > the lock, and we may have both a mapping and be inside try_to_unmap(). > > Is this a problem? On a path with mappings we trylock and so solve the > set_dirty_locked and recursive deadlock issues, and with no mappings > with always dirty the page and avoid data corruption. The problem as I see it is !page->mapping are likely an insignificant minority of userptr; as I think even memfd are essentially shmemfs (or hugetlbfs) and so have mappings. -Chris