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

On Sat, May 10, 2014 at 2:57 AM, Andy Lutomirski [off-list ref] wrote:

On Thu, May 8, 2014 at 10:31 PM, Michael Kerrisk (man-pages)
[off-list ref] wrote:

quoted

On 05/08/2014 10:38 PM, Andy Lutomirski wrote:

quoted

On Thu, May 8, 2014 at 6:29 AM, Michael Kerrisk (man-pages)
[off-list ref] wrote:

quoted

Hi Christoph,

On Thu, May 8, 2014 at 2:50 PM, Christoph Hellwig [off-list ref] wrote:

quoted

On Thu, May 08, 2014 at 11:45:05AM +0200, Michael Kerrisk (man-pages) wrote:

quoted

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?

quoted

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?

quoted

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?

I think so, except for the "on on".

Thanks, Andy. Fixed.

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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