Re: [PATCH] remap_file_pages.2: Not actually useful on real files.

["Michael Kerrisk (man-pages)" <mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

On 05/08/2014 10:38 PM, Andy Lutomirski wrote:
> On Thu, May 8, 2014 at 6:29 AM, Michael Kerrisk (man-pages)
> <mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
>> Hi Christoph,
>>
>> On Thu, May 8, 2014 at 2:50 PM, Christoph Hellwig <hch-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org> wrote:
>>> On Thu, May 08, 2014 at 11:45:05AM +0200, Michael Kerrisk (man-pages) wrote:
>>>> I've applied the above. I then tweaked it a little. Is the following
>>>> okay:
>>>>
>>>> [[
>>>> Since Linux 2.6.23,
>>>> .\" commit 3ee6dafc677a68e461a7ddafc94a580ebab80735
>>>> .BR remap_file_pages ()
>>>> has no performance advantage over
>>>> .BR mmap (2)
>>>> when used on real files:
>>>> on real files it creates a separate VMA for each range.
>>>> It does, however, continue to provide a performance advantage
>>>> for files on memory-based filesystems.
>>>> ]]
>>>
>>> I think "real file" is a very bad term.  What is more real about one
>>> file vs another?  Is NFS less real than XFS, is tmpfs more real than
>>> ramfs?
>>>
>>> I'd reword this more like this:
>>>
>>> Since Linux 2.6.23, remap_file_pages only creates non-linear mappings
>>> on in-memory file systems like tmpfs, hugetlbfs or ramfs.  File systems
>>> with a backing store provide a less efficient emulation.
>>
>> Yes, sounds better to me. Any tweaks you want to add to that, Andy?
>>
>>> I think the whole man page for remap_file_pages is a litt confusing I
>>> have to say, the concept of a VMA is purely kernel internal and doesn't
>>> really have a meaning for applications and thus shouldn't appear in a
>>> man page.
>>
>> I agree it could be better. Do you have a suggested text?
>>
>>> While we're at it:  It seems like we should get rid of the remap_pages
>>> vma operation - it's set by lots of filesystems that can never have
>>> it invoked, and always is set to generic_file_remap_pages anyway.
> 
> Something along the lines of "on filesystems with a backing store,
> remap_file_pages is not much more efficient than using mmap(2) to
> adjust which parts of the file are mapped to which addresses" might
> get the idea across.

So, now I have:

[[
Since Linux 2.6.23,
.\" commit 3ee6dafc677a68e461a7ddafc94a580ebab80735
.BR remap_file_pages ()
creates non-linear mappings only on
on in-memory file systems such as tmpfs, hugetlbfs or ramfs.
On filesystems with a backing store,
.BR remap_file_pages ()
is not much more efficient than using
.BR mmap (2)
to adjust which parts of the file are mapped to which addresses.
]]

Okay?

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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