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=-1.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, HK_RANDOM_FROM,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 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 3C9A2C76186 for ; Wed, 17 Jul 2019 13:46:19 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 1BFC5208C0 for ; Wed, 17 Jul 2019 13:46:19 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1725799AbfGQNqS (ORCPT ); Wed, 17 Jul 2019 09:46:18 -0400 Received: from mga18.intel.com ([134.134.136.126]:9161 "EHLO mga18.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726085AbfGQNqS (ORCPT ); Wed, 17 Jul 2019 09:46:18 -0400 X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from orsmga007.jf.intel.com ([10.7.209.58]) by orsmga106.jf.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 17 Jul 2019 06:46:17 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.64,274,1559545200"; d="scan'208";a="158467995" Received: from esulliva-mobl.ger.corp.intel.com (HELO [10.251.94.109]) ([10.251.94.109]) by orsmga007.jf.intel.com with ESMTP; 17 Jul 2019 06:46:16 -0700 Subject: Re: [Intel-gfx] [PATCH 1/5] drm/i915/userptr: Beware recursive lock_page() To: Chris Wilson , intel-gfx@lists.freedesktop.org Cc: stable@vger.kernel.org References: <20190716124931.5870-1-chris@chris-wilson.co.uk> <156329142200.9436.8651620549785965913@skylake-alporthouse-com> <156336944635.4375.7269371478914847980@skylake-alporthouse-com> <6038b21f-c052-36c5-2d56-72ddeb069097@linux.intel.com> <156337053617.4375.13675276970408492219@skylake-alporthouse-com> From: Tvrtko Ursulin Organization: Intel Corporation UK Plc Message-ID: <951e2751-15d7-9ca8-ef6f-299ba59c47a6@linux.intel.com> Date: Wed, 17 Jul 2019 14:46:15 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.2 MIME-Version: 1.0 In-Reply-To: <156337053617.4375.13675276970408492219@skylake-alporthouse-com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: stable-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: stable@vger.kernel.org On 17/07/2019 14:35, Chris Wilson wrote: > Quoting Tvrtko Ursulin (2019-07-17 14:23:55) >> >> On 17/07/2019 14:17, Chris Wilson wrote: >>> 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). >> >> Normal malloc so anonymous pages? Or you meant everything _apart_ from >> the most basic malloc? > > Aye missed the not. > >>>>> 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. >> >> Better then nothing, no? If easy to do.. > > Actually, I erring on the opposite side. Peeking at mm/ internals does > not bode confidence and feels indefensible. I'd much rather throw my > hands up and say "this is the best we can do with the API provided, > please tell us what we should have done." To which the answer is > probably to not have used gup in the first place :| """ /* * set_page_dirty() is racy if the caller has no reference against * page->mapping->host, and if the page is unlocked. This is because another * CPU could truncate the page off the mapping and then free the mapping. * * Usually, the page _is_ locked, or the caller is a user-space process which * holds a reference on the inode by having an open file. * * In other cases, the page should be locked before running set_page_dirty(). */ int set_page_dirty_lock(struct page *page) """ Could we hold a reference to page->mapping->host while having pages and then would be okay to call plain set_page_dirty? Regards, Tvrtko