mm: kill frontswap

The only user of frontswap is zswap, and has been for a long time.  Have
swap call into zswap directly and remove the indirection.

[[email protected]: remove obsolete comment, per Yosry]
  Link: https://lkml.kernel.org/r/[email protected]
[[email protected]: don't warn if none swapcache folio is passed to zswap_load]
  Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Johannes Weiner <[email protected]>
Signed-off-by: Yin Fengwei <[email protected]>
Acked-by: Konrad Rzeszutek Wilk <[email protected]>
Acked-by: Nhat Pham <[email protected]>
Acked-by: Yosry Ahmed <[email protected]>
Acked-by: Christoph Hellwig <[email protected]>
Cc: Domenico Cerasuolo <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Cc: Vitaly Wool <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: hnaz

Documentation/admin-guide/mm/zswap.rst

@@ -49,7 +49,7 @@ compressed pool.
 Design
 ======
 
-Zswap receives pages for compression through the Frontswap API and is able to
+Zswap receives pages for compression from the swap subsystem and is able to
 evict pages from its own compressed pool on an LRU basis and write them back to
 the backing swap device in the case that the compressed pool is full.
 
@@ -70,19 +70,19 @@ means the compression ratio will always be 2:1 or worse (because of half-full
 zbud pages).  The zsmalloc type zpool has a more complex compressed page
 storage method, and it can achieve greater storage densities.
 
-When a swap page is passed from frontswap to zswap, zswap maintains a mapping
+When a swap page is passed from swapout to zswap, zswap maintains a mapping
 of the swap entry, a combination of the swap type and swap offset, to the zpool
 handle that references that compressed swap page.  This mapping is achieved
 with a red-black tree per swap type.  The swap offset is the search key for the
 tree nodes.
 
-During a page fault on a PTE that is a swap entry, frontswap calls the zswap
-load function to decompress the page into the page allocated by the page fault
-handler.
+During a page fault on a PTE that is a swap entry, the swapin code calls the
+zswap load function to decompress the page into the page allocated by the page
+fault handler.
 
 Once there are no PTEs referencing a swap page stored in zswap (i.e. the count
-in the swap_map goes to 0) the swap code calls the zswap invalidate function,
-via frontswap, to free the compressed entry.
+in the swap_map goes to 0) the swap code calls the zswap invalidate function
+to free the compressed entry.
 
 Zswap seeks to be simple in its policies.  Sysfs attributes allow for one user
 controlled policy:

Documentation/mm/frontswap.rst

@@ -44,7 +44,6 @@ above structured documentation, or deleted if it has served its purpose.
    balance
    damon/index
    free_page_reporting
-   frontswap
    hmm
    hwpoison
    hugetlbfs_reserv