brd in a memdesc world

[Matthew Wilcox <willy@infradead.org>]

Hi Jens,

I'm looking for an architecture-level decision on what the brd driver
should look like once struct page has been shrunk to a minimal size
(more detail at https://kernelnewbies.org/MatthewWilcox/Memdescs )

Currently brd uses page->index as a debugging check.  In the memdesc
future, struct page has no members (you could store a small amount of
information in it, but I'm not willing to commit to more than a few bits).

brd doesn't use anything else from struct page, as far as I can tell.
It just calls kmap_atomic() / __free_page() / flush_dcache_page() (and
it doesn't need to call flush_dcache_page() because you can't mmap the
pages in the brd's array).

Now if you have plans to, eg, support page migration, you're going to need
a bit more infrastructure than just allocating pages, but for what you
have at the moment, just removing the debugging checks that page->index ==
idx would make you entirely compatible with the memdesc future.

Any problem with that?