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

On Mon, Sep 12, 2016 at 3:09 AM, Kirill A. Shutemov
[off-list ref] wrote:

On Sun, Sep 11, 2016 at 10:31:35AM -0700, Dan Williams wrote:

quoted

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 <arnd@arndb.de>
Cc: Andrea Arcangeli <redacted>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
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);

We had few attempts to extand mincore(2) interface/functionality before.
None of them ended up in upsteam.

How this attempt compares to previous?

Not sure, I'm wading into this cold trying to get my pet problem
solved, hence the RFC.

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