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:

quoted

-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.

quoted

+
+	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 ...
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