fs/proc/task_mmu: implement IOCTL to get and optionally clear info about PTEs

The PAGEMAP_SCAN IOCTL on the pagemap file can be used to get or optionally
clear the info about page table entries. The following operations are
supported in this IOCTL:
- Scan the address range and get the memory ranges matching the provided
  criteria. This is performed when the output buffer is specified.
- Write-protect the pages. The PM_SCAN_WP_MATCHING is used to write-protect
  the pages of interest. The PM_SCAN_CHECK_WPASYNC aborts the operation if
  non-Async Write Protected pages are found. The ``PM_SCAN_WP_MATCHING``
  can be used with or without PM_SCAN_CHECK_WPASYNC.
- Both of those operations can be combined into one atomic operation where
  we can get and write protect the pages as well.

Following flags about pages are currently supported:
- PAGE_IS_WPALLOWED - Page has async-write-protection enabled
- PAGE_IS_WRITTEN - Page has been written to from the time it was write protected
- PAGE_IS_FILE - Page is file backed
- PAGE_IS_PRESENT - Page is present in the memory
- PAGE_IS_SWAPPED - Page is in swapped
- PAGE_IS_PFNZERO - Page has zero PFN
- PAGE_IS_HUGE - Page is THP or Hugetlb backed

This IOCTL can be extended to get information about more PTE bits. The
entire address range passed by user [start, end) is scanned until either
the user provided buffer is full or max_pages have been found.

[[email protected]: update it for "mm: hugetlb: add huge page size param to set_huge_pte_at()"]
[[email protected]: fix CONFIG_HUGETLB_PAGE=n warning]
[[email protected]: hide unused pagemap_scan_backout_range() function]
  Link: https://lkml.kernel.org/r/[email protected]
[[email protected]: fix "fs/proc/task_mmu: hide unused pagemap_scan_backout_range() function"]
  Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Muhammad Usama Anjum <[email protected]>
Signed-off-by: Michał Mirosław <[email protected]>
Signed-off-by: Arnd Bergmann <[email protected]>
Signed-off-by: Stephen Rothwell <[email protected]>
Reviewed-by: Andrei Vagin <[email protected]>
Reviewed-by: Michał Mirosław <[email protected]>
Cc: Alex Sierra <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Axel Rasmussen <[email protected]>
Cc: Christian Brauner <[email protected]>
Cc: Cyrill Gorcunov <[email protected]>
Cc: Dan Williams <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: Gustavo A. R. Silva <[email protected]>
Cc: "Liam R. Howlett" <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Michal Miroslaw <[email protected]>
Cc: Mike Rapoport (IBM) <[email protected]>
Cc: Nadav Amit <[email protected]>
Cc: Pasha Tatashin <[email protected]>
Cc: Paul Gofman <[email protected]>
Cc: Peter Xu <[email protected]>
Cc: Shuah Khan <[email protected]>
Cc: Suren Baghdasaryan <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Yang Shi <[email protected]>
Cc: Yun Zhou <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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