[PATCH v2 10/12] ubifs: Fix to add refcount once page is set private

[Zhihao Cheng]

MM defined the rule [1] very clearly that once page was set with PG_private
flag, we should increment the refcount in that page, also main flows like
pageout(), migrate_page() will assume there is one additional page
reference count if page_has_private() returns true. Otherwise, we may
get a BUG in page migration:

  page:0000000080d05b9d refcount:-1 mapcount:0 mapping:000000005f4d82a8
  index:0xe2 pfn:0x14c12
  aops:ubifs_file_address_operations [ubifs] ino:8f1 dentry name:"f30e"
  flags: 0x1fffff80002405(locked|uptodate|owner_priv_1|private|node=0|
  zone=1|lastcpupid=0x1fffff)
  page dumped because: VM_BUG_ON_PAGE(page_count(page) != 0)
  ------------[ cut here ]------------
  kernel BUG at include/linux/page_ref.h:184!
  invalid opcode: 0000 [#1] SMP
  CPU: 3 PID: 38 Comm: kcompactd0 Not tainted 5.15.0-rc5
  RIP: 0010:migrate_page_move_mapping+0xac3/0xe70
  Call Trace:
    ubifs_migrate_page+0x22/0xc0 [ubifs]
    move_to_new_page+0xb4/0x600
    migrate_pages+0x1523/0x1cc0
    compact_zone+0x8c5/0x14b0
    kcompactd+0x2bc/0x560
    kthread+0x18c/0x1e0
    ret_from_fork+0x1f/0x30

The BUG is caused by following process:
PA(cpu 1)                           PB(cpu 2)
ubifs_write_begin
  page = grab_cache_page_write_begin
  (refcnf = 3, for page creation process)
ubifs_write_end
  SetPagePrivate(page)
  unlock_page(page)  // refcnt=3
  put_page(page)
    page_ref_dec_and_test
                                      lock(page)
                                      ...
                                      ubifs_migrate_page
                                        migrate_page_move_mapping
                                          expected_page_refs get 3
                                          (1 + mapping[1] + private[1])
                                          page_ref_freeze  // refcnt = 0
      atomic_dec_and_test(0 - 1 = -1)
                                          page_ref_unfreeze
                                          VM_BUG_ON_PAGE(-1 != 0, page)

Actually zhangjun has tried to fix this problem [2] by recalculating page
refcnt in ubifs_migrate_page(). It's better to follow MM rules [1], because
just like Kirill suggested in [2], we need to check all users of
page_has_private() helper. Like f2fs does in [3], fix it by adding/deleting
refcount when setting/clearing private for a page. BTW, according to [4],
we set 'page->private' as 1 because ubifs just simply SetPagePrivate().
And, [5] provided a common helper to set/clear page private, ubifs can
use this helper following the example of iomap, afs, btrfs, etc.

Jump [6] to find a reproducer.

[1] https://lore.kernel.org/lkml/2b19b3c4-2bc4-15fa-15cc-27a13e5c7af1@xxxxxxx
[2] https://www.spinics.net/lists/linux-mtd/msg04018.html
[3] http://lkml.iu.edu/hypermail/linux/kernel/1903.0/03313.html
[4] https://lore.kernel.org/linux-f2fs-devel/20210422154705.GO3596236@xxxxxxxxxxxxxxxxxxxx
[5] https://lore.kernel.org/all/20200517214718.468-1-guoqing.jiang@xxxxxxxxxxxxxxx
[6] https://bugzilla.kernel.org/show_bug.cgi?id=214961

Fixes: 1e51764a3c2ac0 ("UBIFS: add new flash file system")
Signed-off-by: Zhihao Cheng <chengzhihao1@xxxxxxxxxx>
---
 fs/ubifs/file.c | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/fs/ubifs/file.c b/fs/ubifs/file.c
index 5cfa28cd00cd..6b45a037a047 100644
--- a/fs/ubifs/file.c
+++ b/fs/ubifs/file.c
@@ -570,7 +570,7 @@ static int ubifs_write_end(struct file *file, struct address_space *mapping,
 	}
 
 	if (!PagePrivate(page)) {
-		SetPagePrivate(page);
+		attach_page_private(page, (void *)1);
 		atomic_long_inc(&c->dirty_pg_cnt);
 		__set_page_dirty_nobuffers(page);
 	}
@@ -947,7 +947,7 @@ static int do_writepage(struct page *page, int len)
 		release_existing_page_budget(c);
 
 	atomic_long_dec(&c->dirty_pg_cnt);
-	ClearPagePrivate(page);
+	detach_page_private(page);
 	ClearPageChecked(page);
 
 	kunmap(page);
@@ -1304,7 +1304,7 @@ static void ubifs_invalidatepage(struct page *page, unsigned int offset,
 		release_existing_page_budget(c);
 
 	atomic_long_dec(&c->dirty_pg_cnt);
-	ClearPagePrivate(page);
+	detach_page_private(page);
 	ClearPageChecked(page);
 }
 
@@ -1471,8 +1471,8 @@ static int ubifs_migrate_page(struct address_space *mapping,
 		return rc;
 
 	if (PagePrivate(page)) {
-		ClearPagePrivate(page);
-		SetPagePrivate(newpage);
+		detach_page_private(page);
+		attach_page_private(newpage, (void *)1);
 	}
 
 	if (mode != MIGRATE_SYNC_NO_COPY)
@@ -1496,7 +1496,7 @@ static int ubifs_releasepage(struct page *page, gfp_t unused_gfp_flags)
 		return 0;
 	ubifs_assert(c, PagePrivate(page));
 	ubifs_assert(c, 0);
-	ClearPagePrivate(page);
+	detach_page_private(page);
 	ClearPageChecked(page);
 	return 1;
 }
@@ -1567,7 +1567,7 @@ static vm_fault_t ubifs_vm_page_mkwrite(struct vm_fault *vmf)
 	else {
 		if (!PageChecked(page))
 			ubifs_convert_page_budget(c);
-		SetPagePrivate(page);
+		attach_page_private(page, (void *)1);
 		atomic_long_inc(&c->dirty_pg_cnt);
 		__set_page_dirty_nobuffers(page);
 	}
-- 
2.31.1