Re: [syzbot] [mm?] BUG: Bad page map (8)

[Hillf Danton <hdanton@sina.com>]

On Thu, 18 Jul 2024 15:51:26 -0700
> syzbot found the following issue on:
> 
> HEAD commit:    4d145e3f830b Merge tag 'i2c-for-6.10-rc8' of git://git.ker..
> git tree:       upstream
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1366ab85980000

#syz test https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git  4d145e3f830b

--- x/include/linux/rmap.h
+++ y/include/linux/rmap.h
@@ -327,6 +327,7 @@ static __always_inline void __folio_dup_
 
 	switch (level) {
 	case RMAP_LEVEL_PTE:
+		BUG_ON(atomic_read(&page->_mapcount) < 0);
 		if (!folio_test_large(folio)) {
 			atomic_inc(&page->_mapcount);
 			break;
@@ -419,6 +420,7 @@ static __always_inline int __folio_try_d
 					return -EBUSY;
 		}
 
+		BUG_ON(atomic_read(&page->_mapcount) < 0);
 		if (!folio_test_large(folio)) {
 			if (PageAnonExclusive(page))
 				ClearPageAnonExclusive(page);
--- x/mm/rmap.c
+++ y/mm/rmap.c
@@ -1149,6 +1149,7 @@ static __always_inline unsigned int __fo
 
 	switch (level) {
 	case RMAP_LEVEL_PTE:
+		BUG_ON(atomic_read(&page->_mapcount) < 0);
 		if (!folio_test_large(folio)) {
 			nr = atomic_inc_and_test(&page->_mapcount);
 			break;
@@ -1503,6 +1504,7 @@ static __always_inline void __folio_remo
 
 	switch (level) {
 	case RMAP_LEVEL_PTE:
+		BUG_ON(atomic_read(&page->_mapcount) < 0);
 		if (!folio_test_large(folio)) {
 			nr = atomic_add_negative(-1, &page->_mapcount);
 			break;
--