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
