On Fri, Aug 09, 2019 at 03:16:33AM -0700, Matthew Wilcox wrote: > On Fri, Aug 09, 2019 at 01:03:17PM +0530, Anshuman Khandual wrote: > > Should alloc_gigantic_page() be made available as an interface for general > > use in the kernel. The test module here uses very similar implementation from > > HugeTLB to allocate a PUD aligned memory block. Similar for mm_alloc() which > > needs to be exported through a header. > > Why are you allocating memory at all instead of just using some > known-to-exist PFNs like I suggested? IIUC the issue is that there aren't necessarily known-to-exist PFNs that are sufficiently aligned -- they may not even exist. For example, with 64K pages, a PMD covers 512M. The kernel image is (generally) smaller than 512M, and will be mapped at page granularity. In that case, any PMD entry for a kernel symbol address will point to the PTE level table, and that will only necessarily be page-aligned, as any P?D level table is only necessarily page-aligned. In the same configuration, you could have less than 512M of total memory, and none of this memory is necessarily aligned to 512M. So beyond the PTE level, I don't think you can guarantee a known-to-exist valid PFN. I also believe that synthetic PFNs could fail pfn_valid(), so that might cause us pain too... Thanks, Mark.
