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

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Message-ID: <20160912100910.GC23346@node.shutemov.name>
Date:   Mon, 12 Sep 2016 13:09:10 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     linux-mm@...ck.org, Andrea Arcangeli <aarcange@...hat.com>,
        Xiao Guangrong <guangrong.xiao@...ux.intel.com>,
        Arnd Bergmann <arnd@...db.de>, linux-nvdimm@...ts.01.org,
        linux-api@...r.kernel.org,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [RFC PATCH 1/2] mm, mincore2(): retrieve dax and tlb-size
 attributes of an address range

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@...db.de>
> Cc: Andrea Arcangeli <aarcange@...hat.com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Dave Hansen <dave.hansen@...ux.intel.com>
> Cc: Xiao Guangrong <guangrong.xiao@...ux.intel.com>
> Cc: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> Signed-off-by: Dan Williams <dan.j.williams@...el.com>
> ---
>  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?

-- 
 Kirill A. Shutemov