Re: [PATCH 05/20] mm/migrate: Convert expected_page_refs() to folio_expected_refs()

[Matthew Wilcox <willy@infradead.org>]

On Tue, Jun 07, 2022 at 09:41:57AM -0400, Brian Foster wrote:
> On Mon, Jun 06, 2022 at 09:40:35PM +0100, Matthew Wilcox (Oracle) wrote:
> > -static int expected_page_refs(struct address_space *mapping, struct page *page)
> > +static int folio_expected_refs(struct address_space *mapping,
> > +		struct folio *folio)
> >  {
> > -	int expected_count = 1;
> > +	int refs = 1;
> > +	if (!mapping)
> > +		return refs;
> >  
> > -	if (mapping)
> > -		expected_count += compound_nr(page) + page_has_private(page);
> > -	return expected_count;
> > +	refs += folio_nr_pages(folio);
> > +	if (folio_get_private(folio))
> > +		refs++;
> 
> Why not folio_has_private() (as seems to be used for later
> page_has_private() conversions) here?

We have a horrid confusion that I'm trying to clean up stealthily
without anyone noticing.  I would have gotten away with it too if it
weren't for you pesky kids.

#define PAGE_FLAGS_PRIVATE                              \
        (1UL << PG_private | 1UL << PG_private_2)

static inline int page_has_private(struct page *page)
{
        return !!(page->flags & PAGE_FLAGS_PRIVATE);
}

So what this function is saying is that there is one extra refcount
expected on the struct page if PG_private _or_ PG_private_2 is set.

How are filesystems expected to manage their page's refcount with this
rule?  Increment the refcount when setting PG_private unless
PG_private_2 is already set?  Decrement the refcount when clearing
PG_private_2 unless PG_private is set?

This is garbage.  IMO, PG_private_2 should have no bearing on the page's
refcount.  Only btrfs and the netfs's use private_2 and neither of them
do anything to the refcount when setting/clearing it.  So that's what
I'm implementing here.

> > +
> > +	return refs;;
> 
> Nit: extra ;

Oh, that's where it went ;-)  I had a compile error due to a missing
semicolon at some point, and thought it was just a typo ...