Re: [RFC] /dev/anon

[Davide Libenzi <davidel@xmailserver.org>]

On Wed, 14 Jan 2004, Andrew Morton wrote:

> Davide Libenzi <davidel@xmailserver.org> wrote:
> >
> > On Wed, 14 Jan 2004, Jeff Dike wrote:
> > 
> > > > I thought your goal was to release memory 
> > > > to the host, that's why I proposed sys_madvise(MADV_DONTNEED).
> > > 
> > > It is, I want memory released immediately as though it were clean, and
> > > MADV_DONTNEED doesn't help.
> > 
> > Strange, I didn't notice this before. If you look at the comment in 
> > mm/madvise.c:madvise_dontneed, it advertises that dirty pages are actually 
> > thrown away (that would be what you're actually looking for). But if you 
> > go down to zap_page_range -> unmap_vmas -> unmap_page_range -> 
> > zap_pmd_range -> zap_pte_range, if the page is dirty, set_page_dirty -> 
> > __set_page_dirty_buffers pushes the page into the mapping dirty pages list 
> > and __mark_inode_dirty push the inode inside the superblock dirty list. So 
> > the comment seems to be wrong (I also verified this with a simple program, 
> > and pages are actually flushed).
> > 
> 
> We cannot invalidate the pages due to MADV_DONTNEED: there may be
> freshly-allocated, unwritten file blocks associated with them.  You'd have
> to be playing games with write() amd MAP_SHARED to do this.

That's fine Andrew. It was the comment that looked strange to me. We 
actually do sync dirty pages, while the comment says we throw them away.



- Davide