page_ref tracepoints

[Matthew Wilcox <willy@infradead.org>]

The page reference count tracepoints currently look like this:

                __entry->pfn = page_to_pfn(page);
                __entry->flags = page->flags;
                __entry->count = page_ref_count(page);
                __entry->mapcount = atomic_read(&page->_mapcount);
                __entry->mapping = page->mapping;
                __entry->mt = get_pageblock_migratetype(page);
        TP_printk("pfn=0x%lx flags=%s count=%d mapcount=%d mapping=%p mt=%d val=%d",


Soon, pages will not have a ->mapping, nor a ->mapcount [1].  But they will
still have a refcount, at least for now.  put_page() will move out of
line and look something like this:

void put_page(struct page *page)
{
        unsigned long memdesc = page->memdesc;
        if (memdesc_is_folio(memdesc))
                return folio_put(memdesc_folio(memdesc));
        BUG_ON(memdesc_is_slab(memdesc));
        ... handle other memdesc types here ...
	if (memdesc_is_compound_head(memdesc))
		page = memdesc_head_page(memdesc);

        if (put_page_testzero(page))
                __put_page(page);
}

What I'm thinking is:

 - Define a set of folio_ref_* tracepoints which dump exactly the same info
   as page_ref does today
 - Remove mapping & mapcount from page_ref_* functions.

Other ideas?  I don't use these tracepoints myself; they generate far
too much data to be useful to me.

[1] In case you missed it,
https://lore.kernel.org/linux-mm/Z37pxbkHPbLYnDKn@casper.infradead.org/