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 5A0A4C7618F for ; Wed, 17 Jul 2019 18:09:41 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id CB6A4208C0 for ; Wed, 17 Jul 2019 18:09:40 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727399AbfGQSJk (ORCPT ); Wed, 17 Jul 2019 14:09:40 -0400 Received: from mga18.intel.com ([134.134.136.126]:31113 "EHLO mga18.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727066AbfGQSJk (ORCPT ); Wed, 17 Jul 2019 14:09:40 -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 11:09:39 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.64,275,1559545200"; d="scan'208";a="158539171" 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 11:09:38 -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> <951e2751-15d7-9ca8-ef6f-299ba59c47a6@linux.intel.com> <156337241401.4375.2377981562987470090@skylake-alporthouse-com> From: Tvrtko Ursulin Organization: Intel Corporation UK Plc Message-ID: Date: Wed, 17 Jul 2019 19:09:37 +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: <156337241401.4375.2377981562987470090@skylake-alporthouse-com> Content-Type: text/plain; charset=utf-8; format=flowed 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 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