From a197cf2886a5dc20c0969cf93d4f187713fcb07d Mon Sep 17 00:00:00 2001
From: Baoquan He <bhe@redhat.com>
Date: Sun, 6 Oct 2019 17:58:15 -0700
Subject: [PATCH] memcg: only record foreign writebacks with dirty pages when
 memcg is not disabled

References: git fixes (mm/memcontrol)
Patch-mainline: v5.4-rc3
Git-commit: 08d1d0e6d0a00c6e687201774f3bf61177741e80

In kdump kernel, memcg usually is disabled with 'cgroup_disable=memory'
for saving memory.  Now kdump kernel will always panic when dump vmcore
to local disk:

  BUG: kernel NULL pointer dereference, address: 0000000000000ab8
  Oops: 0000 [#1] SMP NOPTI
  CPU: 0 PID: 598 Comm: makedumpfile Not tainted 5.3.0+ #26
  Hardware name: HPE ProLiant DL385 Gen10/ProLiant DL385 Gen10, BIOS A40 10/02/2018
  RIP: 0010:mem_cgroup_track_foreign_dirty_slowpath+0x38/0x140
  Call Trace:
   __set_page_dirty+0x52/0xc0
   iomap_set_page_dirty+0x50/0x90
   iomap_write_end+0x6e/0x270
   iomap_write_actor+0xce/0x170
   iomap_apply+0xba/0x11e
   iomap_file_buffered_write+0x62/0x90
   xfs_file_buffered_aio_write+0xca/0x320 [xfs]
   new_sync_write+0x12d/0x1d0
   vfs_write+0xa5/0x1a0
   ksys_write+0x59/0xd0
   do_syscall_64+0x59/0x1e0
   entry_SYSCALL_64_after_hwframe+0x44/0xa9

And this will corrupt the 1st kernel too with 'cgroup_disable=memory'.

Via the trace and with debugging, it is pointing to commit 97b27821b485
("writeback, memcg: Implement foreign dirty flushing") which introduced
this regression.  Disabling memcg causes the null pointer dereference at
uninitialized data in function mem_cgroup_track_foreign_dirty_slowpath().

Fix it by returning directly if memcg is disabled, but not trying to
record the foreign writebacks with dirty pages.

Link: http://lkml.kernel.org/r/20190924141928.GD31919@MiWiFi-R3L-srv
Fixes: 97b27821b485 ("writeback, memcg: Implement foreign dirty flushing")
Signed-off-by: Baoquan He <bhe@redhat.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Tejun Heo <tj@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Mel Gorman <mgorman@suse.de>
---
 include/linux/memcontrol.h | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 9b60863429cc..98380779f6d5 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -1264,6 +1264,9 @@ void mem_cgroup_track_foreign_dirty_slowpath(struct page *page,
 static inline void mem_cgroup_track_foreign_dirty(struct page *page,
 						  struct bdi_writeback *wb)
 {
+	if (mem_cgroup_disabled())
+		return;
+
 	if (unlikely(&page->mem_cgroup->css != wb->memcg_css))
 		mem_cgroup_track_foreign_dirty_slowpath(page, wb);
 }