[PATCH V8 0/2] arm64/mm: Enable memory hot remove

[Anshuman Khandual]

This series enables memory hot remove on arm64 after fixing a possible arm64
platform specific kernel page table race condition. This series is based on
linux-next (next-20190920).

Concurrent vmalloc() and hot-remove conflict:

As pointed out earlier on the v5 thread [2] there can be potential conflict
between concurrent vmalloc() and memory hot-remove operation. The problem here
is caused by inadequate locking in vmalloc() which protects installation of a
page table page but not the walk or the leaf entry modification.

Now free_empty_tables() and it's children functions take into account a maximum
possible range on which it operates as a floor-ceiling boundary. This along with
changed free_pxx_table() makes sure that no page table page is freed unless its
fully within the maximum possible range as decided by the caller.

Testing:

Memory hot remove has been tested on arm64 for 4K, 16K, 64K page config
options with all possible CONFIG_ARM64_VA_BITS and CONFIG_PGTABLE_LEVELS
combinations.

Changes in V8:

- Dropped the first patch (memblock_[free|remove] reorder) from the series which
  is no longer needed for arm64 hot-remove enablement and was posted separately
  as (https://patchwork.kernel.org/patch/11146361/)
- Dropped vmalloc-vmemmap detection and subsequent skipping of free_empty_tables()
- Changed free_empty_[pxx]_tables() functions which now accepts a possible maximum
  floor-ceiling address range on which it operates. Also changed free_pxx_table()
  functions to check against required alignment as well as maximum floor-ceiling
  range as another prerequisite before freeing the page table page.
- Dropped remove_pagetable(), instead call it's constituent functions directly

Changes in V7: (https://lkml.org/lkml/2019/9/3/326)

- vmalloc_vmemmap_overlap gets evaluated early during boot for a given config
- free_empty_tables() gets conditionally called based on vmalloc_vmemmap_overlap

Changes in V6: (https://lkml.org/lkml/2019/7/15/36)

- Implemented most of the suggestions from Mark Rutland
- Added <linux/memory_hotplug.h> in ptdump
- remove_pagetable() now has two distinct passes over the kernel page table
- First pass unmap_hotplug_range() removes leaf level entries at all level
- Second pass free_empty_tables() removes empty page table pages
- Kernel page table lock has been dropped completely
- vmemmap_free() does not call freee_empty_tables() to avoid conflict with vmalloc()
- All address range scanning are converted to do {} while() loop
- Added 'unsigned long end' in __remove_pgd_mapping()
- Callers need not provide starting pointer argument to free_[pte|pmd|pud]_table() 
- Drop the starting pointer argument from free_[pte|pmd|pud]_table() functions
- Fetching pxxp[i] in free_[pte|pmd|pud]_table() is wrapped around in READ_ONCE()
- free_[pte|pmd|pud]_table() now computes starting pointer inside the function
- Fixed TLB handling while freeing huge page section mappings at PMD or PUD level
- Added WARN_ON(!page) in free_hotplug_page_range()
- Added WARN_ON(![pm|pud]_table(pud|pmd)) when there is no section mapping

- [PATCH 1/3] mm/hotplug: Reorder memblock_[free|remove]() calls in try_remove_memory()
- Request earlier for separate merger (https://patchwork.kernel.org/patch/10986599/)
- s/__remove_memory/try_remove_memory in the subject line
- s/arch_remove_memory/memblock_[free|remove] in the subject line
- A small change in the commit message as re-order happens now for memblock remove
  functions not for arch_remove_memory()

Changes in V5: (https://lkml.org/lkml/2019/5/29/218)

- Have some agreement [1] over using memory_hotplug_lock for arm64 ptdump
- Change 7ba36eccb3f8 ("arm64/mm: Inhibit huge-vmap with ptdump") already merged
- Dropped the above patch from this series
- Fixed indentation problem in arch_[add|remove]_memory() as per David
- Collected all new Acked-by tags
 
Changes in V4: (https://lkml.org/lkml/2019/5/20/19)

- Implemented most of the suggestions from Mark Rutland
- Interchanged patch [PATCH 2/4] <---> [PATCH 3/4] and updated commit message
- Moved CONFIG_PGTABLE_LEVELS inside free_[pud|pmd]_table()
- Used READ_ONCE() in missing instances while accessing page table entries
- s/p???_present()/p???_none() for checking valid kernel page table entries
- WARN_ON() when an entry is !p???_none() and !p???_present() at the same time
- Updated memory hot-remove commit message with additional details as suggested
- Rebased the series on 5.2-rc1 with hotplug changes from David and Michal Hocko
- Collected all new Acked-by tags

Changes in V3: (https://lkml.org/lkml/2019/5/14/197)
 
- Implemented most of the suggestions from Mark Rutland for remove_pagetable()
- Fixed applicable PGTABLE_LEVEL wrappers around pgtable page freeing functions
- Replaced 'direct' with 'sparse_vmap' in remove_pagetable() with inverted polarity
- Changed pointer names ('p' at end) and removed tmp from iterations
- Perform intermediate TLB invalidation while clearing pgtable entries
- Dropped flush_tlb_kernel_range() in remove_pagetable()
- Added flush_tlb_kernel_range() in remove_pte_table() instead
- Renamed page freeing functions for pgtable page and mapped pages
- Used page range size instead of order while freeing mapped or pgtable pages
- Removed all PageReserved() handling while freeing mapped or pgtable pages
- Replaced XXX_index() with XXX_offset() while walking the kernel page table
- Used READ_ONCE() while fetching individual pgtable entries
- Taken overall init_mm.page_table_lock instead of just while changing an entry
- Dropped previously added [pmd|pud]_index() which are not required anymore
- Added a new patch to protect kernel page table race condition for ptdump
- Added a new patch from Mark Rutland to prevent huge-vmap with ptdump

Changes in V2: (https://lkml.org/lkml/2019/4/14/5)

- Added all received review and ack tags
- Split the series from ZONE_DEVICE enablement for better review
- Moved memblock re-order patch to the front as per Robin Murphy
- Updated commit message on memblock re-order patch per Michal Hocko
- Dropped [pmd|pud]_large() definitions
- Used existing [pmd|pud]_sect() instead of earlier [pmd|pud]_large()
- Removed __meminit and __ref tags as per Oscar Salvador
- Dropped unnecessary 'ret' init in arch_add_memory() per Robin Murphy
- Skipped calling into pgtable_page_dtor() for linear mapping page table
  pages and updated all relevant functions

Changes in V1: (https://lkml.org/lkml/2019/4/3/28)

References:

[1] https://lkml.org/lkml/2019/5/28/584
[2] https://lkml.org/lkml/2019/6/11/709

Anshuman Khandual (2):
  arm64/mm: Hold memory hotplug lock while walking for kernel page table dump
  arm64/mm: Enable memory hot remove

 arch/arm64/Kconfig              |   3 +
 arch/arm64/include/asm/memory.h |   1 +
 arch/arm64/mm/mmu.c             | 305 ++++++++++++++++++++++++++++++++++++++--
 arch/arm64/mm/ptdump_debugfs.c  |   4 +
 4 files changed, 304 insertions(+), 9 deletions(-)

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2.7.4