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    SubjectRe: [PATCH v2 0/9] s390: implement and optimize vmemmap_free()
    On Wed, Jul 22, 2020 at 11:45:49AM +0200, David Hildenbrand wrote:
    > This series is based on the latest s390/features branch [1]. It
    > consolidates vmem_add_range(), vmem_remove_range(), and vmemmap_populate()
    > into a single, recursive page table walker. It then implements
    > vmemmap_free() and optimizes it by
    > - Freeing empty page tables (also done for vmem_remove_range()).
    > - Handling cases where the vmemmap of a section does not fill huge pages
    > completely (e.g., sizeof(struct page) == 56).
    >
    > vmemmap_free() is currently never used, unless adiing standby memory fails
    > (unlikely). This is relevant for virtio-mem, which adds/removes memory
    > in memory block/section granularity (always removes memory in the same
    > granularity it added it).
    >
    > I gave this a proper test with my virtio-mem prototype (which I will share
    > in the near future), both with 56 byte memmap per page and 64 byte memmap
    > per page, with and without huge page support. In both cases, removing
    > memory (routed through arch_remove_memory()) will result in
    > - all populated vmemmap pages to get removed/freed
    > - all applicable page tables for the vmemmap getting removed/freed
    > - all applicable page tables for the idendity mapping getting removed/freed
    > Unfortunately, I don't have access to bigger and z/VM (esp. dcss)
    > environments.
    >
    > This is the basis for real memory hotunplug support for s390x and should
    > complete my journey to s390x vmem/vmemmap code for now
    >
    > What needs double-checking is tlb flushing. AFAIKS, as there are no valid
    > accesses, doing a single range flush at the end is sufficient, both when
    > removing vmemmap pages and the idendity mapping.
    >
    > [1] https://git.kernel.org/pub/scm/linux/kernel/git/s390/linux.git/commit/?h=features
    >
    > v1 -> v2:
    > - Convert to a single page table walker named "modify_pagetable()", with
    > two helper functions "add_pagetable()" and "remove_pagetable().
    >
    > David Hildenbrand (9):
    > s390/vmem: rename vmem_add_mem() to vmem_add_range()
    > s390/vmem: consolidate vmem_add_range() and vmem_remove_range()
    > s390/vmemmap: extend modify_pagetable() to handle vmemmap
    > s390/vmemmap: cleanup when vmemmap_populate() fails
    > s390/vmemmap: take the vmem_mutex when populating/freeing
    > s390/vmem: cleanup empty page tables
    > s390/vmemmap: fallback to PTEs if mapping large PMD fails
    > s390/vmemmap: remember unused sub-pmd ranges
    > s390/vmemmap: avoid memset(PAGE_UNUSED) when adding consecutive
    > sections
    >
    > arch/s390/mm/vmem.c | 637 ++++++++++++++++++++++++++++++--------------
    > 1 file changed, 442 insertions(+), 195 deletions(-)

    Series applied, thank you!

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