From 08d1d0e6d0a00c6e687201774f3bf61177741e80 Mon Sep 17 00:00:00 2001 From: Baoquan He Date: Sun, 6 Oct 2019 17:58:15 -0700 Subject: [PATCH] memcg: only record foreign writebacks with dirty pages when memcg is not disabled 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 Acked-by: Michal Hocko Cc: Johannes Weiner Cc: Jan Kara Cc: Tejun Heo Cc: Jens Axboe Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- 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); }