mm/thp: decrease nr_thps in file's mapping on THP split

Decrease nr_thps counter in file's mapping to ensure that the page cache
won't be dropped excessively on file write access if page has been
already split.

I've tried a test scenario running a big binary, kernel remaps it with
THPs, then force a THP split with /sys/kernel/debug/split_huge_pages.
During any further open of that binary with O_RDWR or O_WRITEONLY kernel
drops page cache for it, because of non-zero thps counter.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Marek Szyprowski <[email protected]>
Fixes: 09d91cd ("mm,thp: avoid writes to file with THP in pagecache")
Fixes: 06d3eff ("mm/thp: fix node page state in split_huge_page_to_list()")
Acked-by: Matthew Wilcox (Oracle) <[email protected]>
Reviewed-by: Yang Shi <[email protected]>
Cc: <[email protected]>
Cc: Song Liu <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: "Kirill A . Shutemov" <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Hillf Danton <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: William Kucharski <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/huge_memory.c

@@ -2700,12 +2700,14 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
 		if (mapping) {
 			int nr = thp_nr_pages(head);
 
-			if (PageSwapBacked(head))
+			if (PageSwapBacked(head)) {
 				__mod_lruvec_page_state(head, NR_SHMEM_THPS,
 							-nr);
-			else
+			} else {
 				__mod_lruvec_page_state(head, NR_FILE_THPS,
 							-nr);
+				filemap_nr_thps_dec(mapping);
+			}
 		}
 
 		__split_huge_page(page, list, end);