On Wed, Mar 17, 2021 at 06:20:32PM +0100, Christoph Hellwig wrote:

quoted

+static inline void attach_page_private(struct page *page, void *data)
+{
+	attach_folio_private((struct folio *)page, data);
+}
+
+static inline void *detach_page_private(struct page *page)
+{
+	return detach_folio_private((struct folio *)page);
+}

I hate these open code casts.  Can't we have a single central
page_to_folio helper, which could also grow a debug check (maybe
under a new config option) to check that it really is called on a
head page?

Some of that is already there.  We have page_folio() which is the
page_to_folio() helper you're asking for.  And folio_flags() (which is
called *all the time*) contains
        VM_BUG_ON_PGFLAGS(PageTail(page), page);
Someone passing around a tail pointer cast to a folio is not going to
get very far, assuming CONFIG_DEBUG_VM_PGFLAGS is enabled (most distros
don't, but I do when I'm testing anything THPish).

These helpers aren't going to live for very long ... I expect to have
all filesystems which use attach/detach page private converted to folios
pretty soon.  Certainly before any of them _use_ multi-page folios.

Anyway, the simple thing to do is just to use page_folio() here and eat
the cost of calling compound_head() on something we're certain is an
order-0 page.  It only defers the win of removing the compound_head()
call; it doesn't preclude it.  And it means we're not setting a bad
example here (there really shouldn't be any casts from pages to folios,
except in the folio allocator, which uses the page allocator and then
casts what _must be_ a non-tail page to a folio).