mm/khugepaged: remove reuse_swap_page() usage

reuse_swap_page() currently indicates if we can write to an anon page
without COW.  A COW is required if the page is shared by multiple
processes (either already mapped or via swap entries) or if there is
concurrent writeback that cannot tolerate concurrent page modifications.

However, in the context of khugepaged we're not actually going to write to
a read-only mapped page, we'll copy the page content to our newly
allocated THP and map that THP writable.  All we have to make sure is that
the read-only mapped page we're about to copy won't get reused by another
process sharing the page, otherwise, page content would get modified.  But
that is already guaranteed via multiple mechanisms (e.g., holding a
reference, holding the page lock, removing the rmap after copying the
page).

The swapcache handling was introduced in commit 1035921 ("mm:
incorporate read-only pages into transparent huge pages") and it sounds
like it merely wanted to mimic what do_swap_page() would do when trying to
map a page obtained via the swapcache writable.

As that logic is unnecessary, let's just remove it, removing the last user
of reuse_swap_page().

Link: https://lkml.kernel.org/r/20220131162940.210846-7-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Yang Shi <shy828301@gmail.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: David Rientjes <rientjes@google.com>
Cc: Don Dutile <ddutile@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Jann Horn <jannh@google.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Liang Zhang <zhangliang5@huawei.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Mike Rapoport <rppt@linux.ibm.com>
Cc: Nadav Amit <nadav.amit@gmail.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Shakeel Butt <shakeelb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Author: davidhildenbrand

include/trace/events/huge_memory.h

@@ -29,7 +29,6 @@
 	EM( SCAN_VMA_NULL,		"vma_null")			\
 	EM( SCAN_VMA_CHECK,		"vma_check_failed")		\
 	EM( SCAN_ADDRESS_RANGE,		"not_suitable_address_range")	\
-	EM( SCAN_SWAP_CACHE_PAGE,	"page_swap_cache")		\
 	EM( SCAN_DEL_PAGE_LRU,		"could_not_delete_page_from_lru")\
 	EM( SCAN_ALLOC_HUGE_PAGE_FAIL,	"alloc_huge_page_failed")	\
 	EM( SCAN_CGROUP_CHARGE_FAIL,	"ccgroup_charge_failed")	\

mm/khugepaged.c

@@ -46,7 +46,6 @@ enum scan_result {
 	SCAN_VMA_NULL,
 	SCAN_VMA_CHECK,
 	SCAN_ADDRESS_RANGE,
-	SCAN_SWAP_CACHE_PAGE,
 	SCAN_DEL_PAGE_LRU,
 	SCAN_ALLOC_HUGE_PAGE_FAIL,
 	SCAN_CGROUP_CHARGE_FAIL,
@@ -683,16 +682,6 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
 			result = SCAN_PAGE_COUNT;
 			goto out;
 		}
-		if (!pte_write(pteval) && PageSwapCache(page) &&
-				!reuse_swap_page(page)) {
-			/*
-			 * Page is in the swap cache and cannot be re-used.
-			 * It cannot be collapsed into a THP.
-			 */
-			unlock_page(page);
-			result = SCAN_SWAP_CACHE_PAGE;
-			goto out;
-		}
 
 		/*
 		 * Isolate the page to avoid collapsing an hugepage