Re: page_ref tracepoints

[Alistair Popple <apopple@nvidia.com>]

On Thu, Jan 23, 2025 at 07:55:24PM +0000, Matthew Wilcox wrote:
> 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.

I'm afraid I don't have any specific ideas but in the past I have used
these tracepoints mostly to debug issues around what is holding a pin on a
page and therefore preventing some operation, usually migration. For that
page_ref_count(page) and page->_mapcount were the most important fields, with
the latter required to determine the "expected" refcount.

The ->mapping field was less interesting to me when I have used these
tracepoints.

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