linux-kernel - Re: [PATCH 1/2] mm: mark remap_file

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Date:	Thu, 12 Jun 2014 11:44:01 +0200
From:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH 1/2] mm: mark remap_file_pages() syscall as deprecated

Hi Kirill,

On Thu, Jun 12, 2014 at 11:40 AM, Kirill A. Shutemov
<kirill.shutemov@...ux.intel.com> wrote:
> Michael Kerrisk wrote:
>> Hi Kirill,
>>
>> On Thu, May 8, 2014 at 2:41 PM, Kirill A. Shutemov
>> <kirill.shutemov@...ux.intel.com> wrote:
>> > The remap_file_pages() system call is used to create a nonlinear mapping,
>> > that is, a mapping in which the pages of the file are mapped into a
>> > nonsequential order in memory. The advantage of using remap_file_pages()
>> > over using repeated calls to mmap(2) is that the former approach does not
>> > require the kernel to create additional VMA (Virtual Memory Area) data
>> > structures.
>> >
>> > Supporting of nonlinear mapping requires significant amount of non-trivial
>> > code in kernel virtual memory subsystem including hot paths. Also to get
>> > nonlinear mapping work kernel need a way to distinguish normal page table
>> > entries from entries with file offset (pte_file). Kernel reserves flag in
>> > PTE for this purpose. PTE flags are scarce resource especially on some CPU
>> > architectures. It would be nice to free up the flag for other usage.
>> >
>> > Fortunately, there are not many users of remap_file_pages() in the wild.
>> > It's only known that one enterprise RDBMS implementation uses the syscall
>> > on 32-bit systems to map files bigger than can linearly fit into 32-bit
>> > virtual address space. This use-case is not critical anymore since 64-bit
>> > systems are widely available.
>> >
>> > The plan is to deprecate the syscall and replace it with an emulation.
>> > The emulation will create new VMAs instead of nonlinear mappings. It's
>> > going to work slower for rare users of remap_file_pages() but ABI is
>> > preserved.
>> >
>> > One side effect of emulation (apart from performance) is that user can hit
>> > vm.max_map_count limit more easily due to additional VMAs. See comment for
>> > DEFAULT_MAX_MAP_COUNT for more details on the limit.
>>
>> Best to CC linux-api@
>> (https://www.kernel.org/doc/man-pages/linux-api-ml.html) on patches
>> like this, as well as the man-pages maintainer, so that something goes
>> into the man page. I added the following into the man page:
>>
>>        Note:  this  system  call  is (since Linux 3.16) deprecated and
>>        will eventually be replaced by a  slower  in-kernel  emulation.
>>        Those  few  applications  that use this system call should con‐
>>        sider migrating to alternatives.
>>
>> Okay?
>
> Yep. Looks okay to me.

Thanks for checking.

Cheers,

Michael

-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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