mm: prohibit the last subpage from reusing the entire large folio

In a Copy-on-Write (CoW) scenario, the last subpage will reuse the entire
large folio, resulting in the waste of (nr_pages - 1) pages.  This wasted
memory remains allocated until it is either unmapped or memory reclamation
occurs.

The following small program can serve as evidence of this behavior

 main()
 {
 #define SIZE 1024 * 1024 * 1024UL
         void *p = malloc(SIZE);
         memset(p, 0x11, SIZE);
         if (fork() == 0)
                 _exit(0);
         memset(p, 0x12, SIZE);
         printf("done\n");
         while(1);
 }

For example, using a 1024KiB mTHP by:
 echo always > /sys/kernel/mm/transparent_hugepage/hugepages-1024kB/enabled

(1) w/o the patch, it takes 2GiB,

Before running the test program,
 / # free -m
                total        used        free      shared  buff/cache   available
 Mem:            5754          84        5692           0          17        5669
 Swap:              0           0           0

 / # /a.out &
 / # done

After running the test program,
 / # free -m
                 total        used        free      shared  buff/cache   available
 Mem:            5754        2149        3627           0          19        3605
 Swap:              0           0           0

(2) w/ the patch, it takes 1GiB only,

Before running the test program,
 / # free -m
                 total        used        free      shared  buff/cache   available
 Mem:            5754          89        5687           0          17        5664
 Swap:              0           0           0

 / # /a.out &
 / # done

After running the test program,
 / # free -m
                total        used        free      shared  buff/cache   available
 Mem:            5754        1122        4655           0          17        4632
 Swap:              0           0           0

This patch migrates the last subpage to a small folio and immediately
returns the large folio to the system. It benefits both memory availability
and anti-fragmentation.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Barry Song <[email protected]>
Acked-by: David Hildenbrand <[email protected]>
Cc: Ryan Roberts <[email protected]>
Cc: Lance Yang <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

mm/memory.c

@@ -3498,6 +3498,16 @@ static vm_fault_t wp_page_shared(struct vm_fault *vmf, struct folio *folio)
 static bool wp_can_reuse_anon_folio(struct folio *folio,
 				    struct vm_area_struct *vma)
 {
+	/*
+	 * We could currently only reuse a subpage of a large folio if no
+	 * other subpages of the large folios are still mapped. However,
+	 * let's just consistently not reuse subpages even if we could
+	 * reuse in that scenario, and give back a large folio a bit
+	 * sooner.
+	 */
+	if (folio_test_large(folio))
+		return false;
+
 	/*
 	 * We have to verify under folio lock: these early checks are
 	 * just an optimization to avoid locking the folio and freeing