Re: [PATCH] Documentation/mm: Describe folios in physical_memory.rst

[Matthew Wilcox <willy@infradead.org>]

On Fri, Dec 15, 2023 at 01:00:12PM +0100, Fabio M. De Francesco wrote:
> +A folio is a physically, virtually and logically contiguous set of bytes.
> +It is a power-of-two in size, and it is aligned to that same power-of-two.
> +It is at least as large as %PAGE_SIZE. If it is in the page cache, it is
> +at a file offset which is a multiple of that power-of-two. It may be
> +mapped into userspace at an address which is at an arbitrary page offset,
> +but its kernel virtual address is aligned to its size.

This text is verbatim from include/linux/mm_types.h.  It seems sad
to have kernel-doc and then replicate it in an rst file.

> +As Matthew Wilcox explains in his introduction to folios, the need for

oof, no, don't mention my name.

> +`struct folio` arises mostly to address issues with the use of compound
> +pages. It is often unclear whether a function operates on an individual
> +page, or an entire compound page.
> +
> +"A function which has a `struct page` pointer argument might be
> +expecting a head or base page and will BUG if given a tail page. It might
> +work with any kind of page and operate on %PAGE_SIZE bytes. It might work
> +with any kind of page and operate on page_size() bytes if given a head
> +page but %PAGE_SIZE bytes if given a base or tail page. It might operate
> +on page_size() bytes if passed a head or tail page. We have examples of
> +all of these today.".
> +
> +A pointer to folio points to a page that is never a tail page. It
> +represents an entire compound page. Therefore, there is no need to call
> +compound_head() to get a pointer to the head. Folios has eliminted the
> +need to unnecessary calls and has avoided bugs related to the misuse of
> +pages passed to functions. Furthermore, the inline compound_head() makes
> +the kernel bigger and slows things down.
> +
> +The folio APIs are described in the "Memory Management APIs" document.

This was exactly the kind of documentation I was hoping you wouldn't
write ;-(  It's documentation that makes sense today, but won't in five
years time.

We want to say something like,

A folio represents a single memory allocation.  It may be composed of
several pages ...