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=-1.5 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 CB963C49ED6 for ; Wed, 11 Sep 2019 11:31:34 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id A9D792089F for ; Wed, 11 Sep 2019 11:31:34 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727565AbfIKLbe (ORCPT ); Wed, 11 Sep 2019 07:31:34 -0400 Received: from mga07.intel.com ([134.134.136.100]:23372 "EHLO mga07.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726702AbfIKLbe (ORCPT ); Wed, 11 Sep 2019 07:31:34 -0400 X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from fmsmga007.fm.intel.com ([10.253.24.52]) by orsmga105.jf.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 11 Sep 2019 04:31:33 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.64,493,1559545200"; d="scan'208";a="185795379" Received: from avrahamr-mobl1.ger.corp.intel.com (HELO [10.252.3.203]) ([10.252.3.203]) by fmsmga007.fm.intel.com with ESMTP; 11 Sep 2019 04:31:31 -0700 Subject: Re: [Intel-gfx] [PATCH 1/5] drm/i915/userptr: Beware recursive lock_page() To: Chris Wilson , Lionel Landwerlin , 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> <951e2751-15d7-9ca8-ef6f-299ba59c47a6@linux.intel.com> <156337241401.4375.2377981562987470090@skylake-alporthouse-com> <4a90e8f9-694c-8dea-45b6-e5ea5677df64@intel.com> <156803716592.27961.18000112287811684297@skylake-alporthouse-com> From: Tvrtko Ursulin Organization: Intel Corporation UK Plc Message-ID: <90d744ec-17ac-b8d1-e9c0-d34c16adcd4f@linux.intel.com> Date: Wed, 11 Sep 2019 12:31:32 +0100 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: <156803716592.27961.18000112287811684297@skylake-alporthouse-com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: stable-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: stable@vger.kernel.org On 09/09/2019 14:52, Chris Wilson wrote: > Quoting Lionel Landwerlin (2019-07-26 14:38:40) >> On 17/07/2019 21:09, Tvrtko Ursulin wrote: >>> >>> On 17/07/2019 15:06, Chris Wilson wrote: >>>> Quoting Tvrtko Ursulin (2019-07-17 14:46:15) >>>>> >>>>> 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? >>>> >>>> We would then be hitting the warnings in ext4 for unlocked pages again. >>> >>> Ah true.. >>> >>>> Essentially the argument is whether or not that warn is valid, to >>>> which I >>>> think requires inner knowledge of vfs + ext4. To hold a reference on the >>>> host would require us tracking page->mapping (reasonable since we >>>> already hooked into mmu and so will get an invalidate + fresh gup on >>>> any changes), plus iterating over all to acquire the extra reference if >>>> applicable -- and I have no idea what the side-effects of that would be. >>>> Could well be positive side-effects. Just feels like wandering even >>>> further off the beaten path without a map. Good news hmm is just around >>>> the corner (which will probably prohibit this use-case) :| >>> >>> ... can we reach out to someone more knowledgeable in mm matters to >>> recommend us what to do? >>> >>> Regards, >>> >>> Tvrtko >> >> >> Just a reminder to not let this slip. >> We run into userptr bugs in CI quite regularly. > > Remind away. Revert or trylock, there doesn't seem to be a good answer. Rock and a hard place. Data corruption for userptr users (with either trylock or no lock) or a deadlock (with the lock). I honestly can't decide what is worse. Tiny preference to deadlock rather than silent corruption. Misguided? Don't know really.. Regards, Tvrtko