On Wed, 2010-12-01 at 11:23 -0800, Hugh Dickins wrote:
quoted text
> On Wed, 1 Dec 2010, Trond Myklebust wrote:
> > On Wed, 2010-12-01 at 08:17 -0800, Linus Torvalds wrote:
> > > include/linux/fs.h |    1 +
> > >  mm/vmscan.c        |    3 +++
> > >  2 files changed, 4 insertions(+), 0 deletions(-)
> > > 
> > > diff --git a/include/linux/fs.h b/include/linux/fs.h
> > > index c9e06cc..090f0ea 100644
> > > --- a/include/linux/fs.h
> > > +++ b/include/linux/fs.h
> > > @@ -602,6 +602,7 @@ struct address_space_operations {
> > >         sector_t (*bmap)(struct address_space *, sector_t);
> > >         void (*invalidatepage) (struct page *, unsigned long);
> > >         int (*releasepage) (struct page *, gfp_t);
> > > +       void (*freepage)(struct page *);
> > >         ssize_t (*direct_IO)(int, struct kiocb *, const struct iovec
> > > *iov,
> > >                         loff_t offset, unsigned long nr_segs);
> > >         int (*get_xip_mem)(struct address_space *, pgoff_t, int,
> > > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > > index d31d7ce..1accb01 100644
> > > --- a/mm/vmscan.c
> > > +++ b/mm/vmscan.c
> > > @@ -499,6 +499,9 @@ static int __remove_mapping(struct address_space
> > > *mapping, struct page *page)
> > >                 mem_cgroup_uncharge_cache_page(page);
> > >         }
> > >  
> > > +       if (mapping->a_ops->freepage)
> > > +               mapping->a_ops->freepage(page);
> > 
> > Hmm... Looking again at the problem, it appears that the same callback
> > needs to be added to truncate_complete_page() and
> > invalidate_complete_page2(). Otherwise we end up in a situation where
> > the page can sometimes be removed from the page cache without calling
> > freepage().
> > 
> > > +
> > >         return 1;
> > >  
> > >  cannot_free:
> 
> Yes, I was wondering quite how we would define this ->freepage thing,
> if it gets called from one place that removes from page cache and not
> from others.
> 
> Another minor problem with it: it would probably need to take the
> struct address_space *mapping as arg as well as struct page *page:
> because by this time page->mapping has been reset to NULL.
> 
> But I'm not at all keen on adding a calllback in this very special
> frozen-to-0-references place: please let's not do it without an okay
> from Nick Piggin (now Cc'ed).
> 
> I agree completely with what Linus said originally about how the
> page cannot be freed while there's a reference to it, and it should
> be possible to work this without your additional page locks.
> 
> Your ->releasepage should be able to judge whether the page is likely
> (not certain) to be freed - page_count 3? 1 for the page cache, 1 for
> the page_private reference, 1 for vmscan's reference, I think.  Then
> it can mark !PageUptodate and proceed with freeing the stuff you had
> allocated, undo page_has_private and its reference, and return 1 (or
> return 0 if it decides to hold on to the page).

That is very brittle. I'd prefer not to have to scan linux-mm every week
in order to find out if someone changed the page_count.

However, while reading Documentation/filesystems/vfs.txt (in order to
add documentation for freepage) I was surprised to read that the
->releasepage() is itself supposed to be allowed to actually remove the
page from the address space if it so desires.

Looking at the actual code in shrink_page_list() and friends I can't see
how that can possibly fail to break things, but if it were true, then
that might enable us to call remove_mapping() in order to safely free
the page before it gets cleared.

Cheers
  Trond

-- 
Trond Myklebust
Linux NFS client maintainer

NetApp
Trond.Myklebust@netapp.com
www.netapp.com

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