Re: set_page_dirty races (was: Re: [patch 2/4] vfs: add set_page_dirty_notag)

[Nick Piggin <npiggin@suse.de>]

On Tue, Feb 17, 2009 at 12:39:32PM +0100, Peter Zijlstra wrote:
> On Tue, 2009-02-17 at 12:25 +0100, Nick Piggin wrote:
> 
> > Introduce put_user_pages function.
> > 
> > In order to have more flexibility to deal with issues surrounding
> > get_user_pages difficulties[*], introduce put_user_pages function
> > intended to release pages acquired by get_user_pages. For now, just
> > do the regular put_page thing. If all callers are converted, it could
> > be used to help with such races. In the meantime, it will actually
> > serve as a small extra piece of documentation for the code.
> > 
> > [*] eg. get_user_pages caller can bypass page_mkwrite calls into the
> >     filesystem to notify of page dirty activity if the page gets cleaned
> >     before the caller calls its final set_page_dirty).
> 
> Hmm, if we want to distinguish between .write=1 and .write=0, we would
> have to pass .write to pup too, right?

Doh, yeah. I hand edited the patch to put that parameter in, but quilt
refresh must have outsmarted me!

If nobody thinks it is insane, I'll resend to Andrew in a new thread.

> > ---
> >  include/linux/mm.h |    1 +
> >  mm/memory.c        |   13 ++++++++++++-
> >  2 files changed, 13 insertions(+), 1 deletion(-)
> > 
> > Index: linux-2.6/include/linux/mm.h
> > ===================================================================
> > --- linux-2.6.orig/include/linux/mm.h
> > +++ linux-2.6/include/linux/mm.h
> > @@ -826,6 +826,7 @@ extern int access_process_vm(struct task
> >  int get_user_pages(struct task_struct *tsk, struct mm_struct *mm, unsigned long start,
> >  		int len, int write, int force, struct page **pages, struct vm_area_struct **vmas);
> >  
> > +void put_user_pages(struct page **pages, int nr);
> >  extern int try_to_release_page(struct page * page, gfp_t gfp_mask);
> >  extern void do_invalidatepage(struct page *page, unsigned long offset);
> >  
> > Index: linux-2.6/mm/memory.c
> > ===================================================================
> > --- linux-2.6.orig/mm/memory.c
> > +++ linux-2.6/mm/memory.c
> > @@ -1370,9 +1370,20 @@ int get_user_pages(struct task_struct *t
> >  				start, len, flags,
> >  				pages, vmas);
> >  }
> > -
> >  EXPORT_SYMBOL(get_user_pages);
> >  
> > +/*
> > + * put_user_pages should be used to release pages acquired with get_user_pages.
> > + */
> > +void put_user_pages(struct page **pages, int nr)
> > +{
> > +	int i;
> > +
> > +	for (i = 0; i < nr; i++)
> > +		put_page(pages[i]);
> > +}
> > +EXPORT_SYMBOL(put_user_pages);
> > +
> >  pte_t *get_locked_pte(struct mm_struct *mm, unsigned long addr,
> >  			spinlock_t **ptl)
> >  {
> >