[PATCH 5.14 153/169] mm/thp: decrease nr_thps in files mapping on THP split

[Greg Kroah-Hartman]

From: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>

[ Upstream commit 1ca7554d05ac038c98271f8968ed821266ecaa9c ]

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/20211012120237.2600-1-m.szyprowski@xxxxxxxxxxx
Signed-off-by: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
Fixes: 09d91cda0e82 ("mm,thp: avoid writes to file with THP in pagecache")
Fixes: 06d3eff62d9d ("mm/thp: fix node page state in split_huge_page_to_list()")
Acked-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
Reviewed-by: Yang Shi <shy828301@xxxxxxxxx>
Cc: <sfoon.kim@xxxxxxxxxxx>
Cc: Song Liu <songliubraving@xxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxxx>
Cc: "Kirill A . Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Hillf Danton <hdanton@xxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: William Kucharski <william.kucharski@xxxxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 mm/huge_memory.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index afff3ac87067..163c2da2a654 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2724,12 +2724,14 @@ 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);
-- 
2.33.0