Re: [PATCH net-next 04/12] mm: Make the page_frag_cache allocator use multipage folios

[David Howells <dhowells@redhat.com>]

Alexander H Duyck <alexander.duyck@gmail.com> wrote:

> Also I have some concerns about going from page to folio as it seems
> like the folio_alloc setups the transparent hugepage destructor instead
> of using the compound page destructor. I would think that would slow
> down most users as it looks like there is a spinlock that is taken in
> the hugepage destructor that isn't there in the compound page
> destructor.

Note that this code is going to have to move to folios[*] at some point.
"Old-style" compound pages are going to go away, I believe.  Matthew Wilcox
and the mm folks are on a drive towards simplifying memory management,
formalising chunks larger than a single page - with the ultimate aim of
reducing the page struct to a single, typed pointer.

So, take, for example, a folio: As I understand it, this will no longer
overlay struct page, but rather will become a single, dynamically-allocated
struct that covers a pow-of-2 number of pages.  A contiguous subset of page
structs will point at it.

However, rather than using a folio, we could define a "page fragment" memory
type.  Rather than having all the flags and fields to be found in struct
folio, it could have just the set to be found in page_frag_cache.

David

[*] It will be possible to have some other type than "folio".  See "struct
slab" in mm/slab.h for example.  struct slab corresponds to a set of pages
and, in the future, a number of struct pages will point at it.