Re: [RFC PATCH 1/2] mm, mincore2(): retrieve dax and tlb-size attributes of an address range

As evidenced by this bug report [1], userspace libraries are interested
in whether a mapping is DAX mapped, i.e. no intervening page cache.
Rather than using the ambiguous VM_MIXEDMAP flag in smaps, provide an
explicit "is dax" indication as a new flag in the page vector populated
by mincore.

There are also cases, particularly for testing and validating a
configuration to know the hardware mapping geometry of the pages in a
given process address range.  Consider filesystem-dax where a
configuration needs to take care to align partitions and block
allocations before huge page mappings might be used, or
anonymous-transparent-huge-pages where a process is opportunistically
assigned large pages.  mincore2() allows these configurations to be
surveyed and validated.

The implementation takes advantage of the unused bits in the per-page
byte returned for each PAGE_SIZE extent of a given address range.  The
new format of each vector byte is:

(TLB_SHIFT - PAGE_SHIFT) << 2 | vma_is_dax() << 1 | page_present

[1]: https://lkml.org/lkml/2016/9/7/61

Cc: Arnd Bergmann <redacted>
Cc: Andrea Arcangeli <redacted>
Cc: Andrew Morton <redacted>
Cc: Dave Hansen <redacted>
Cc: Xiao Guangrong <redacted>
Cc: Kirill A. Shutemov <redacted>
Signed-off-by: Dan Williams <redacted>
---
 include/linux/syscalls.h               |    2 +
 include/uapi/asm-generic/mman-common.h |    3 +
 kernel/sys_ni.c                        |    1 
 mm/mincore.c                           |  126 +++++++++++++++++++++++++-------
 4 files changed, 104 insertions(+), 28 deletions(-)

diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index d02239022bd0..4aa2ee7e359a 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h

@@ -467,6 +467,8 @@ asmlinkage long sys_munlockall(void);
 asmlinkage long sys_madvise(unsigned long start, size_t len, int behavior);
 asmlinkage long sys_mincore(unsigned long start, size_t len,
 				unsigned char __user * vec);
+asmlinkage long sys_mincore2(unsigned long start, size_t len,
+				unsigned char __user * vec, int flags);