Re: [PATCH v2 4/5] mm: Add zap_skip_check_mapping() helper

[Peter Xu <peterx@redhat.com>]

On Fri, Sep 03, 2021 at 10:58:53AM +1000, Alistair Popple wrote:
> On Friday, 3 September 2021 6:18:19 AM AEST Peter Xu wrote:
> > Use the helper for the checks.  Rename "check_mapping" into "zap_mapping"
> > because "check_mapping" looks like a bool but in fact it stores the mapping
> > itself.  When it's set, we check the mapping (it must be non-NULL).  When it's
> > cleared we skip the check, which works like the old way.
> >
> > Move the duplicated comments to the helper too.
> > 
> > Signed-off-by: Peter Xu <peterx@redhat.com>
> > ---
> >  include/linux/mm.h | 15 ++++++++++++++-
> >  mm/memory.c        | 29 ++++++-----------------------
> >  2 files changed, 20 insertions(+), 24 deletions(-)
> > 
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index 69259229f090..81e402a5fbc9 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -1720,10 +1720,23 @@ extern void user_shm_unlock(size_t, struct ucounts *);
> >   * Parameter block passed down to zap_pte_range in exceptional cases.
> >   */
> >  struct zap_details {
> > -	struct address_space *check_mapping;	/* Check page->mapping if set */
> > +	struct address_space *zap_mapping;	/* Check page->mapping if set */
> >  	struct page *single_page;		/* Locked page to be unmapped */
> >  };
> >  
> > +/*
> > + * We set details->zap_mappings when we want to unmap shared but keep private
> > + * pages. Return true if skip zapping this page, false otherwise.
> > + */
> > +static inline bool
> > +zap_skip_check_mapping(struct zap_details *details, struct page *page)
> > +{
> > +	if (!details || !page)
> > +		return false;
> > +
> > +	return details->zap_mapping != page_rmapping(page);
> 
> Shouldn't this check still be
> details->zap_mapping && details->zap_mapping != page_rmapping(page)?
> 
> Previously we wouldn't skip zapping pages if even_cows == true (ie.
> details->check_mapping == NULL). With this change the check when
> even_cows == true becomes NULL != page_rmapping(page). Doesn't this mean we
> will now skip zapping any pages with a mapping when even_cows == true?

Yes I think so.  Thanks for pointing that out, Alistair, I'll fix in v3.

But frankly I really think we should simply have that flag I used to introduce.
It'll make everything much clearer.

-- 
Peter Xu