Re: [PATCH] mm/dev_pfn: Exclude MEMORY_DEVICE_PRIVATE while computing virtual address
On 05/21/2019 01:03 AM, Dan Williams wrote: > On Mon, May 20, 2019 at 12:27 PM Jerome Glisse 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 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.
Re: [PATCH] mm/dev_pfn: Exclude MEMORY_DEVICE_PRIVATE while computing virtual address
On Mon, May 20, 2019 at 12:27 PM Jerome Glisse 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 > > > 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.
Re: [PATCH] mm/dev_pfn: Exclude MEMORY_DEVICE_PRIVATE while computing virtual address
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 > > 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. Cheers, Jérôme
Re: [PATCH] mm/dev_pfn: Exclude MEMORY_DEVICE_PRIVATE while computing virtual address
On Sun, May 19, 2019 at 10:37 PM Anshuman Khandual wrote: > > On 05/18/2019 03:20 AM, Andrew Morton wrote: > > On Fri, 17 May 2019 16:08:34 +0530 Anshuman Khandual > > 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. +1 to killing it, since there has been a paucity of 'unsigned long pfn' code path conversions to 'pfn_t', and it continues to go unused. > > But first, let's fix it so that if someone brings it back, they bring > > back a non-buggy version. Not sure this can be solved without a rethink of who owns the virtual address space corresponding to MEMORY_DEVICE_PRIVATE, and clawing back some of the special-ness of HMM. > > 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. MEMORY_DEVICE_PRIVATE semantics were part of the package of the initial HMM submission that landed in the kernel without an upstream user. While pfn_t_to_virt() also does not have an upstream user it was at least modeled after the existing pfn_to_virt() api to allow for future 'unsigned long pfn' to 'pfn_t' conversions. As for what a fix might look like, it seems to me that we should try to unify 'pfn_t' and 'hmm_pfn's. I don't see why 'hmm_pfn's need to exist as their own concept vs trying consume more flag space out of pfn_t. That would at least allow the pfn_t_has_page() helper to detect the HMM case.
Re: [PATCH] mm/dev_pfn: Exclude MEMORY_DEVICE_PRIVATE while computing virtual address
On 05/18/2019 03:20 AM, Andrew Morton wrote: > On Fri, 17 May 2019 16:08:34 +0530 Anshuman Khandual > 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.
Re: [PATCH] mm/dev_pfn: Exclude MEMORY_DEVICE_PRIVATE while computing virtual address
On Fri, 17 May 2019 16:08:34 +0530 Anshuman Khandual 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. So... what (would be) the user-visible runtime effects of this change?
