memcg: fix a crash in wb_workfn when a device disappears

[ Upstream commit 68f23b89067fdf187763e75a56087550624fdbee ]

Without memcg, there is a one-to-one mapping between the bdi and
bdi_writeback structures.  In this world, things are fairly
straightforward; the first thing bdi_unregister() does is to shutdown
the bdi_writeback structure (or wb), and part of that writeback ensures
that no other work queued against the wb, and that the wb is fully
drained.

With memcg, however, there is a one-to-many relationship between the bdi
and bdi_writeback structures; that is, there are multiple wb objects
which can all point to a single bdi.  There is a refcount which prevents
the bdi object from being released (and hence, unregistered).  So in
theory, the bdi_unregister() *should* only get called once its refcount
goes to zero (bdi_put will drop the refcount, and when it is zero,
release_bdi gets called, which calls bdi_unregister).

Unfortunately, del_gendisk() in block/gen_hd.c never got the memo about
the Brave New memcg World, and calls bdi_unregister directly.  It does
this without informing the file system, or the memcg code, or anything
else.  This causes the root wb associated with the bdi to be
unregistered, but none of the memcg-specific wb's are shutdown.  So when
one of these wb's are woken up to do delayed work, they try to
dereference their wb->bdi->dev to fetch the device name, but
unfortunately bdi->dev is now NULL, thanks to the bdi_unregister()
called by del_gendisk().  As a result, *boom*.

Fortunately, it looks like the rest of the writeback path is perfectly
happy with bdi->dev and bdi->owner being NULL, so the simplest fix is to
create a bdi_dev_name() function which can handle bdi->dev being NULL.
This also allows us to bulletproof the writeback tracepoints to prevent
them from dereferencing a NULL pointer and crashing the kernel if one is
tracing with memcg's enabled, and an iSCSI device dies or a USB storage
stick is pulled.

The most common way of triggering this will be hotremoval of a device
while writeback with memcg enabled is going on.  It was triggering
several times a day in a heavily loaded production environment.

Google Bug Id: 145475544

Link: https://lore.kernel.org/r/20191227194829.150110-1-tytso@mit.edu
Link: http://lkml.kernel.org/r/20191228005211.163952-1-tytso@mit.edu
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: Chris Mason <clm@fb.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 384f95e1936dd07241c9615a8f071aaa86df763e..fde277be26420b2ee4e78e9a96b2007396b816e9 100644 (file)
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -1965,7 +1965,7 @@ void wb_workfn(struct work_struct *work)
                                                struct bdi_writeback, dwork);
        long pages_written;
 
-       set_worker_desc("flush-%s", dev_name(wb->bdi->dev));
+       set_worker_desc("flush-%s", bdi_dev_name(wb->bdi));
        current->flags |= PF_SWAPWRITE;
 
        if (likely(!current_is_workqueue_rescuer() ||

diff --git a/include/linux/backing-dev.h b/include/linux/backing-dev.h
index 012adec975433125243ce68a71a4f799550c93d5..c947b293805476a295f9e7aba920ee57ee1aa347 100644 (file)
--- a/include/linux/backing-dev.h
+++ b/include/linux/backing-dev.h
@@ -13,6 +13,7 @@
 #include <linux/fs.h>
 #include <linux/sched.h>
 #include <linux/blkdev.h>
+#include <linux/device.h>
 #include <linux/writeback.h>
 #include <linux/blk-cgroup.h>
 #include <linux/backing-dev-defs.h>
@@ -493,4 +494,13 @@ static inline int bdi_rw_congested(struct backing_dev_info *bdi)
                                  (1 << WB_async_congested));
 }
 
+extern const char *bdi_unknown_name;
+
+static inline const char *bdi_dev_name(struct backing_dev_info *bdi)
+{
+       if (!bdi || !bdi->dev)
+               return bdi_unknown_name;
+       return dev_name(bdi->dev);
+}
+
 #endif /* _LINUX_BACKING_DEV_H */

diff --git a/include/trace/events/writeback.h b/include/trace/events/writeback.h
index a8066afb4b0ef21858a27f7b9e1c29f50f755ee9..cb2a5016247affb6aa38a478f520019d6ab77052 100644 (file)
--- a/include/trace/events/writeback.h
+++ b/include/trace/events/writeback.h
@@ -66,8 +66,8 @@ TRACE_EVENT(writeback_dirty_page,
 
        TP_fast_assign(
                strscpy_pad(__entry->name,
-                           mapping ? dev_name(inode_to_bdi(mapping->host)->dev) : "(unknown)",
-                           32);
+                           bdi_dev_name(mapping ? inode_to_bdi(mapping->host) :
+                                        NULL), 32);
                __entry->ino = mapping ? mapping->host->i_ino : 0;
                __entry->index = page->index;
        ),
@@ -96,8 +96,7 @@ DECLARE_EVENT_CLASS(writeback_dirty_inode_template,
                struct backing_dev_info *bdi = inode_to_bdi(inode);
 
                /* may be called for files on pseudo FSes w/ unregistered bdi */
-               strscpy_pad(__entry->name,
-                           bdi->dev ? dev_name(bdi->dev) : "(unknown)", 32);
+               strscpy_pad(__entry->name, bdi_dev_name(bdi), 32);
                __entry->ino            = inode->i_ino;
                __entry->state          = inode->i_state;
                __entry->flags          = flags;
@@ -177,7 +176,7 @@ DECLARE_EVENT_CLASS(writeback_write_inode_template,
 
        TP_fast_assign(
                strscpy_pad(__entry->name,
-                           dev_name(inode_to_bdi(inode)->dev), 32);
+                           bdi_dev_name(inode_to_bdi(inode)), 32);
                __entry->ino            = inode->i_ino;
                __entry->sync_mode      = wbc->sync_mode;
                __entry->cgroup_ino     = __trace_wbc_assign_cgroup(wbc);
@@ -220,9 +219,7 @@ DECLARE_EVENT_CLASS(writeback_work_class,
                __field(unsigned int, cgroup_ino)
        ),
        TP_fast_assign(
-               strscpy_pad(__entry->name,
-                           wb->bdi->dev ? dev_name(wb->bdi->dev) :
-                           "(unknown)", 32);
+               strscpy_pad(__entry->name, bdi_dev_name(wb->bdi), 32);
                __entry->nr_pages = work->nr_pages;
                __entry->sb_dev = work->sb ? work->sb->s_dev : 0;
                __entry->sync_mode = work->sync_mode;
@@ -275,7 +272,7 @@ DECLARE_EVENT_CLASS(writeback_class,
                __field(unsigned int, cgroup_ino)
        ),
        TP_fast_assign(
-               strscpy_pad(__entry->name, dev_name(wb->bdi->dev), 32);
+               strscpy_pad(__entry->name, bdi_dev_name(wb->bdi), 32);
                __entry->cgroup_ino = __trace_wb_assign_cgroup(wb);
        ),
        TP_printk("bdi %s: cgroup_ino=%u",
@@ -298,7 +295,7 @@ TRACE_EVENT(writeback_bdi_register,
                __array(char, name, 32)
        ),
        TP_fast_assign(
-               strscpy_pad(__entry->name, dev_name(bdi->dev), 32);
+               strscpy_pad(__entry->name, bdi_dev_name(bdi), 32);
        ),
        TP_printk("bdi %s",
                __entry->name
@@ -323,7 +320,7 @@ DECLARE_EVENT_CLASS(wbc_class,
        ),
 
        TP_fast_assign(
-               strscpy_pad(__entry->name, dev_name(bdi->dev), 32);
+               strscpy_pad(__entry->name, bdi_dev_name(bdi), 32);
                __entry->nr_to_write    = wbc->nr_to_write;
                __entry->pages_skipped  = wbc->pages_skipped;
                __entry->sync_mode      = wbc->sync_mode;
@@ -374,7 +371,7 @@ TRACE_EVENT(writeback_queue_io,
                __field(unsigned int,   cgroup_ino)
        ),
        TP_fast_assign(
-               strncpy_pad(__entry->name, dev_name(wb->bdi->dev), 32);
+               strscpy_pad(__entry->name, bdi_dev_name(wb->bdi), 32);
                __entry->older  = dirtied_before;
                __entry->age    = (jiffies - dirtied_before) * 1000 / HZ;
                __entry->moved  = moved;
@@ -459,7 +456,7 @@ TRACE_EVENT(bdi_dirty_ratelimit,
        ),
 
        TP_fast_assign(
-               strscpy_pad(__entry->bdi, dev_name(wb->bdi->dev), 32);
+               strscpy_pad(__entry->bdi, bdi_dev_name(wb->bdi), 32);
                __entry->write_bw       = KBps(wb->write_bandwidth);
                __entry->avg_write_bw   = KBps(wb->avg_write_bandwidth);
                __entry->dirty_rate     = KBps(dirty_rate);
@@ -524,7 +521,7 @@ TRACE_EVENT(balance_dirty_pages,
 
        TP_fast_assign(
                unsigned long freerun = (thresh + bg_thresh) / 2;
-               strscpy_pad(__entry->bdi, dev_name(wb->bdi->dev), 32);
+               strscpy_pad(__entry->bdi, bdi_dev_name(wb->bdi), 32);
 
                __entry->limit          = global_wb_domain.dirty_limit;
                __entry->setpoint       = (global_wb_domain.dirty_limit +
@@ -585,7 +582,7 @@ TRACE_EVENT(writeback_sb_inodes_requeue,
 
        TP_fast_assign(
                strscpy_pad(__entry->name,
-                           dev_name(inode_to_bdi(inode)->dev), 32);
+                           bdi_dev_name(inode_to_bdi(inode)), 32);
                __entry->ino            = inode->i_ino;
                __entry->state          = inode->i_state;
                __entry->dirtied_when   = inode->dirtied_when;
@@ -659,7 +656,7 @@ DECLARE_EVENT_CLASS(writeback_single_inode_template,
 
        TP_fast_assign(
                strscpy_pad(__entry->name,
-                           dev_name(inode_to_bdi(inode)->dev), 32);
+                           bdi_dev_name(inode_to_bdi(inode)), 32);
                __entry->ino            = inode->i_ino;
                __entry->state          = inode->i_state;
                __entry->dirtied_when   = inode->dirtied_when;

diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 6fa31754eadd99d85d504aa993ce1e8721947a79..f5a5e9f82b221464b152c90e032f430dce0b2330 100644 (file)
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -19,6 +19,7 @@ struct backing_dev_info noop_backing_dev_info = {
 EXPORT_SYMBOL_GPL(noop_backing_dev_info);
 
 static struct class *bdi_class;
+const char *bdi_unknown_name = "(unknown)";
 
 /*
  * bdi_lock protects updates to bdi_list. bdi_list has RCU reader side