Re: Folio discussion recap

[Ira Weiny]

On Tue, Sep 21, 2021 at 11:18:52PM +0100, Matthew Wilcox wrote:

...

> +/**
> + * page_slab - Converts from page to slab.
> + * @p: The page.
> + *
> + * This function cannot be called on a NULL pointer.  It can be called
> + * on a non-slab page; the caller should check is_slab() to be sure
> + * that the slab really is a slab.
> + *
> + * Return: The slab which contains this page.
> + */
> +#define page_slab(p)		(_Generic((p),				\
> +	const struct page *:	(const struct slab *)_compound_head(p), \
> +	struct page *:		(struct slab *)_compound_head(p)))
> +
> +static inline bool is_slab(struct slab *slab)
> +{
> +	return test_bit(PG_slab, &slab->flags);
> +}
> +

I'm sorry, I don't have a dog in this fight and conceptually I think folios are
a good idea...

But for this work, having a call which returns if a 'struct slab' really is a
'struct slab' seems odd and well, IMHO, wrong.  Why can't page_slab() return
NULL if there is no slab containing that page?

Ira