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

[Dan Williams]

On Mon, Sep 12, 2016 at 3:09 AM, Kirill A. Shutemov
<kirill@xxxxxxxxxxxxx> wrote:
> On Sun, Sep 11, 2016 at 10:31:35AM -0700, Dan Williams wrote:
>> 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@xxxxxxxx>
>> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
>> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
>> Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
>> Cc: Xiao Guangrong <guangrong.xiao@xxxxxxxxxxxxxxx>
>> Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
>> Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
>> ---
>>  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.