mm/zsmalloc: add zpdesc memory descriptor for zswap.zpool

mm/zsmalloc: add zpdesc memory descriptor for zswap.zpool

Patch series "Add zpdesc memory descriptor for zswap.zpool", v9.

This patch series introduces a new memory descriptor for zswap.zpool that
currently overlaps with struct page for now.  This is part of the effort
to reduce the size of struct page and to enable dynamic allocation of
memory descriptors [1].

This series does not bloat anything for zsmalloc and no functional change
is intended (except for using zpdesc and folios).

In the near future, the removal of page->index from struct page [2] will
be addressed and the project also depends on this patch series.

Thanks to everyone got involved in this series, especially, Alex who's
been pushing it forward this year.

[1] https://lore.kernel.org/linux-mm/ZvRKzKizOfEWBtJp@casper.infradead.org
[2] https://lore.kernel.org/linux-mm/Z09hOy-UY9KC8WMb@casper.infradead.org

This patch (of 18):

The 1st patch introduces new memory descriptor zpdesc and renames
zspage.first_page to zspage.first_zpdesc, with no functional change.

We removed the comment about PG_owner_priv_1 since it is no longer used
after commit a41ec880aa7b ("zsmalloc: move huge compressed obj from page
to zspage").

[rdunlap@infradead.org: fix function parameter kernel-doc notation]
Link: https://lkml.kernel.org/r/20250111063305.911010-1-rdunlap@infradead.org
[42.hyeyoo@gmail.com: rework comments a little bit]
Link: https://lkml.kernel.org/r/20241216150450.1228021-1-42.hyeyoo@gmail.com
Link: https://lkml.kernel.org/r/20241216150450.1228021-2-42.hyeyoo@gmail.com
Originally-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Signed-off-by: Alex Shi <alexs@kernel.org>
Signed-off-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Tested-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Alex Shi <alexs@kernel.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Vishal Moola (Oracle) <vishal.moola@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>