Re: [PATCH 3/3] mlock: avoid dirtying pages and triggering writeback

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 17 Nov 2010 18:52:30 -0500
"Ted Ts'o" <tytso@mit.edu> wrote:

> On Thu, Nov 18, 2010 at 10:11:43AM +1100, Dave Chinner wrote:
> > I don't think ->page_mkwrite can be worked around - we need that to
> > be called on the first write fault of any mmap()d page to ensure it
> > is set up correctly for writeback.  If we don't get write faults
> > after the page is mlock()d, then we need the ->page_mkwrite() call
> > during the mlock() call.
> 
> OK, so I'm not an mm hacker, so maybe I'm missing something.  Could
> part of this be fixed by simply sending the write faults for
> mlock()'ed pages, so page_mkwrite() gets called when the page is
> dirtied.  Seems like a real waste to have the file system pre-allocate
> all of the blocks for a mlock()'ed region.  Why does mlock() have to
> result in the write faults getting suppressed when the page is
> actually dirtied?

Yup, I don't think it would be too bad to take a minor fault each time
an mlocked page transitions from clean->dirty.

In fact we should already be doing that, after the mlocked page gets
written back by kupdate?  Hope so!