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

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Date:	Thu, 12 Jun 2014 12:40:14 +0300 (EEST)
From:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:	Michael Kerrisk <mtk.manpages@...il.com>
Cc:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	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>,
	Michael Kerrisk-manpages <mtk.manpages@...il.com>
Subject: Re: [PATCH 1/2] mm: mark remap_file_pages() syscall as deprecated

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.

-- 
 Kirill A. Shutemov
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