On 04/03/2019 11:27 PM, Robin Murphy wrote: > On 03/04/2019 18:32, Logan Gunthorpe wrote: >> >> >> On 2019-04-02 10:30 p.m., Anshuman Khandual wrote: >>> Memory removal from an arch perspective involves tearing down two different >>> kernel based mappings i.e vmemmap and linear while releasing related page >>> table pages allocated for the physical memory range to be removed. >>> >>> Define a common kernel page table tear down helper remove_pagetable() which >>> can be used to unmap given kernel virtual address range. In effect it can >>> tear down both vmemap or kernel linear mappings. This new helper is called >>> from both vmemamp_free() and ___remove_pgd_mapping() during memory removal. >>> The argument 'direct' here identifies kernel linear mappings. >>> >>> Vmemmap mappings page table pages are allocated through sparse mem helper >>> functions like vmemmap_alloc_block() which does not cycle the pages through >>> pgtable_page_ctor() constructs. Hence while removing it skips corresponding >>> destructor construct pgtable_page_dtor(). >>> >>> While here update arch_add_mempory() to handle __add_pages() failures by >>> just unmapping recently added kernel linear mapping. Now enable memory hot >>> remove on arm64 platforms by default with ARCH_ENABLE_MEMORY_HOTREMOVE. >>> >>> This implementation is overall inspired from kernel page table tear down >>> procedure on X86 architecture. >> >> I've been working on very similar things for RISC-V. In fact, I'm >> currently in progress on a very similar stripped down version of >> remove_pagetable(). (Though I'm fairly certain I've done a bunch of >> stuff wrong.) >> >> Would it be possible to move this work into common code that can be used >> by all arches? Seems like, to start, we should be able to support both >> arm64 and RISC-V... and maybe even x86 too. >> >> I'd be happy to help integrate and test such functions in RISC-V. I am more inclined towards consolidating remove_pagetable() across platforms like arm64 and RISC-V (probably others). But there are clear distinctions between user page table and kernel page table tear down process. > > Indeed, I had hoped we might be able to piggyback off generic code for this anyway, > given that we have generic pagetable code which knows how to free process pagetables, > and kernel pagetables are also pagetables. But there are differences. To list some * Freeing mapped and pagetable pages - Memory hot remove deals with both vmemmap and linear mappings - Selectively call pgtable_page_dtor() for linear mappings (arch specific) - Not actually freeing PTE|PMD|PUD mapped pages for linear mappings - Freeing mapped pages for vmemap mappings * TLB shootdown - User page table process uses mmu_gather mechanism for TLB flush - Kernel page table tear down can do with less TLB flush invocations - Dont have to care about flush deferral etc * THP and HugeTLB - Kernel page table tear down procedure does not have to understand THP or HugeTLB - Though it has to understand possible arch specific special block mappings - Specific kernel linear mappings on arm64 - PUD|PMD|CONT_PMD|CONT_PTE large page mappings - Specific vmemmap mappings on arm64 - PMD large or PTE mappings -User page table tear down procedure needs to understand THP and HugeTLB * Page table locking - Kernel procedure locks init_mm.page_table_lock while clearing an individual entry - Kernel procedure does not have to worry about mmap_sem * ZONE_DEVICE struct vmem_altmap - Kernel page table tear down procedure needs to accommodate 'struct vmem_altmap' when vmemmap mappings are created with pages allocated from 'struct vmem_altmap' (ZONE_DEVICE) rather than buddy allocator or memblock.
