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

[Fabio M. De Francesco]

On Friday, 15 December 2023 15:36:21 CET Matthew Wilcox wrote:
> 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.

Actually, I took this text from the private email you sent me. I thought you 
were asking to use exactly this words. And so I acted accordingly to what it 
seemed to me you had suggested. 

Furthermore I had forgotten that these words are in kernel-doc exactly because 
I copy-pasted from your email.

OK. I can explain what a folio is by using different words and elaborating a 
bit.

> > +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.

I wanted to explain why you introduced folios. If you think that the 
historical perspective is not what future developers will need in the next 5 
years, I can think of something else. 

> We want to say something like,
> 
> A folio represents a single memory allocation.  It may be composed of
> several pages ...

Ah, OK. I think I got it.

Thanks for your comments.

Fabio