From mboxrd@z Thu Jan 1 00:00:00 1970 From: Tvrtko Ursulin Subject: Re: [Intel-gfx] [PATCH 1/5] drm/i915/userptr: Beware recursive lock_page() Date: Wed, 17 Jul 2019 14:23:55 +0100 Message-ID: <6038b21f-c052-36c5-2d56-72ddeb069097@linux.intel.com> References: <20190716124931.5870-1-chris@chris-wilson.co.uk> <156329142200.9436.8651620549785965913@skylake-alporthouse-com> <156336944635.4375.7269371478914847980@skylake-alporthouse-com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <156336944635.4375.7269371478914847980@skylake-alporthouse-com> Content-Language: en-US Sender: stable-owner@vger.kernel.org To: Chris Wilson , intel-gfx@lists.freedesktop.org Cc: stable@vger.kernel.org List-Id: intel-gfx@lists.freedesktop.org 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? >>> 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.. Regards, Tvrtko