linux-kernel - Re: Folio discussion recap

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Date:   Wed, 22 Sep 2021 17:45:15 -0700
From:   Ira Weiny <ira.weiny@...el.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Kent Overstreet <kent.overstreet@...il.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Christoph Hellwig <hch@...radead.org>,
        David Howells <dhowells@...hat.com>
Subject: Re: Folio discussion recap

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