linux-kernel - Re: remap_file

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Message-ID: <alpine.LSU.2.11.1405202112520.5142@eggly.anvils>
Date:	Tue, 20 May 2014 21:32:47 -0700 (PDT)
From:	Hugh Dickins <hughd@...gle.com>
To:	Kenny Simpson <theonetruekenny@...il.com>
cc:	"Kirill A. Shutemov" <kirill@...temov.name>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Michal Hocko <mhocko@...e.cz>, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Dave Jones <davej@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Armin Rigo <arigo@...es.org>
Subject: Re: remap_file_pages() use

On Tue, 20 May 2014, Kenny Simpson wrote:

> I might need a gentle nudge with a clue stick...
> checking against latest git tree it looks as though most common
> filesystem types do support remap_file_pages.
> 
> I just wrote a simple test case and it worked on my 3.13-based ubuntu
> 14.04 system on an ext4 filesystem.

It is all very confusing, yes.

When Kirill said disk-backed files don't support remap_file_pages since
commit 3ee6dafc677a, he was meaning that that they do not support it
with a special nonlinear vma; but the remap_file_pages syscall emulates
the layout for them with separate linear vmas instead.

Confusingly, these filesystems opt in to this emulation by pointing
their remap_pages method to generic_file_remap_pages - code which is
then never used for them!  tmpfs is the only filesystem (having no
page_mkwrite) which actually passes through that code.

You can understand why there's some enthusiasm for cleaning this up :)

Hugh
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