On 05/21/2019 01:03 AM, Dan Williams wrote: > On Mon, May 20, 2019 at 12:27 PM Jerome Glisse <jglisse@xxxxxxxxxx> wrote: >> >> On Mon, May 20, 2019 at 11:07:38AM +0530, Anshuman Khandual wrote: >>> On 05/18/2019 03:20 AM, Andrew Morton wrote: >>>> On Fri, 17 May 2019 16:08:34 +0530 Anshuman Khandual <anshuman.khandual@xxxxxxx> wrote: >>>> >>>>> The presence of struct page does not guarantee linear mapping for the pfn >>>>> physical range. Device private memory which is non-coherent is excluded >>>>> from linear mapping during devm_memremap_pages() though they will still >>>>> have struct page coverage. Just check for device private memory before >>>>> giving out virtual address for a given pfn. >>>> >>>> I was going to give my standard "what are the user-visible runtime >>>> effects of this change?", but... >>>> >>>>> All these helper functions are all pfn_t related but could not figure out >>>>> another way of determining a private pfn without looking into it's struct >>>>> page. pfn_t_to_virt() is not getting used any where in mainline kernel.Is >>>>> it used by out of tree drivers ? Should we then drop it completely ? >>>> >>>> Yeah, let's kill it. >>>> >>>> But first, let's fix it so that if someone brings it back, they bring >>>> back a non-buggy version. >>> >>> Makes sense. >>> >>>> >>>> So... what (would be) the user-visible runtime effects of this change? >>> >>> I am not very well aware about the user interaction with the drivers which >>> hotplug and manage ZONE_DEVICE memory in general. Hence will not be able to >>> comment on it's user visible runtime impact. I just figured this out from >>> code audit while testing ZONE_DEVICE on arm64 platform. But the fix makes >>> the function bit more expensive as it now involve some additional memory >>> references. >> >> A device private pfn can never leak outside code that does not understand it >> So this change is useless for any existing users and i would like to keep the >> existing behavior ie never leak device private pfn. > > The issue is that only an HMM expert might know that such a pfn can > never leak, in other words the pfn concept from a code perspective is > already leaked / widespread. Ideally any developer familiar with a pfn > and the core-mm pfn helpers need only worry about pfn semantics > without being required to go audit HMM users. Agreed.
