Merge branch 'for-5.17/struct-slab' into for-linus

Series "Separate struct slab from struct page" v4

This is originally an offshoot of the folio work by Matthew. One of the more
complex parts of the struct page definition are the parts used by the slab
allocators. It would be good for the MM in general if struct slab were its own
data type, and it also helps to prevent tail pages from slipping in anywhere.
As Matthew requested in his proof of concept series, I have taken over the
development of this series, so it's a mix of patches from him (often modified
by me) and my own.

One big difference is the use of coccinelle to perform the relatively trivial
parts of the conversions automatically and at once, instead of a larger number
of smaller incremental reviewable steps. Thanks to Julia Lawall and Luis
Chamberlain for all their help!

Another notable difference is (based also on review feedback) I don't represent
with a struct slab the large kmalloc allocations which are not really a slab,
but use page allocator directly. When going from an object address to a struct
slab, the code tests first folio slab flag, and only if it's set it converts to
struct slab. This makes the struct slab type stronger.

Finally, although Matthew's version didn't use any of the folio work, the
initial support has been merged meanwhile so my version builds on top of it
where appropriate. This eliminates some of the redundant compound_head()
being performed e.g. when testing the slab flag.

To sum up, after this series, struct page fields used by slab allocators are
moved from struct page to a new struct slab, that uses the same physical
storage. The availability of the fields is further distinguished by the
selected slab allocator implementation. The advantages include:

- Similar to folios, if the slab is of order > 0, struct slab always is
  guaranteed to be the head page. Additionally it's guaranteed to be an actual
  slab page, not a large kmalloc. This removes uncertainty and potential for
  bugs.
- It's not possible to accidentally use fields of the slab implementation that's
  not configured.
- Other subsystems cannot use slab's fields in struct page anymore (some
  existing non-slab usages had to be adjusted in this series), so slab
  implementations have more freedom in rearranging them in the struct slab.

Link: https://lore.kernel.org/all/20220104001046.12263-1-vbabka@suse.cz/