Re: [PATCH V2 1/2] arm64/mm: Fix pfn_valid() for ZONE_DEVICE based memory

[Will Deacon]

On Tue, Feb 02, 2021 at 12:32:15PM +0000, Will Deacon wrote:
> On Tue, Feb 02, 2021 at 09:41:53AM +0530, Anshuman Khandual wrote:
> > pfn_valid() validates a pfn but basically it checks for a valid struct page
> > backing for that pfn. It should always return positive for memory ranges
> > backed with struct page mapping. But currently pfn_valid() fails for all
> > ZONE_DEVICE based memory types even though they have struct page mapping.
> > 
> > pfn_valid() asserts that there is a memblock entry for a given pfn without
> > MEMBLOCK_NOMAP flag being set. The problem with ZONE_DEVICE based memory is
> > that they do not have memblock entries. Hence memblock_is_map_memory() will
> > invariably fail via memblock_search() for a ZONE_DEVICE based address. This
> > eventually fails pfn_valid() which is wrong. memblock_is_map_memory() needs
> > to be skipped for such memory ranges. As ZONE_DEVICE memory gets hotplugged
> > into the system via memremap_pages() called from a driver, their respective
> > memory sections will not have SECTION_IS_EARLY set.
> > 
> > Normal hotplug memory will never have MEMBLOCK_NOMAP set in their memblock
> > regions. Because the flag MEMBLOCK_NOMAP was specifically designed and set
> > for firmware reserved memory regions. memblock_is_map_memory() can just be
> > skipped as its always going to be positive and that will be an optimization
> > for the normal hotplug memory. Like ZONE_DEVICE based memory, all normal
> > hotplugged memory too will not have SECTION_IS_EARLY set for their sections
> > 
> > Skipping memblock_is_map_memory() for all non early memory sections would
> > fix pfn_valid() problem for ZONE_DEVICE based memory and also improve its
> > performance for normal hotplug memory as well.
> 
> Hmm. Although I follow your logic, this does seem to rely on an awful lot of
> assumptions to continue to hold true as the kernel evolves. In particular,
> how do we ensure that early sections are always fully backed with

Sorry, typo here:       ^^^ should be *non-early* sections.

Will