queue-4.4/writeback-synchronize-sync-2-against-cgroup-writeback-membership-switches.patch

   1 From 7fc5854f8c6efae9e7624970ab49a1eac2faefb1 Mon Sep 17 00:00:00 2001
   2 From: Tejun Heo <tj@kernel.org>
   3 Date: Tue, 12 Dec 2017 08:38:30 -0800
   4 Subject: writeback: synchronize sync(2) against cgroup writeback membership switches
   5
   6 From: Tejun Heo <tj@kernel.org>
   7
   8 commit 7fc5854f8c6efae9e7624970ab49a1eac2faefb1 upstream.
   9
  10 sync_inodes_sb() can race against cgwb (cgroup writeback) membership
  11 switches and fail to writeback some inodes.  For example, if an inode
  12 switches to another wb while sync_inodes_sb() is in progress, the new
  13 wb might not be visible to bdi_split_work_to_wbs() at all or the inode
  14 might jump from a wb which hasn't issued writebacks yet to one which
  15 already has.
  16
  17 This patch adds backing_dev_info->wb_switch_rwsem to synchronize cgwb
  18 switch path against sync_inodes_sb() so that sync_inodes_sb() is
  19 guaranteed to see all the target wbs and inodes can't jump wbs to
  20 escape syncing.
  21
  22 v2: Fixed misplaced rwsem init.  Spotted by Jiufei.
  23
  24 Signed-off-by: Tejun Heo <tj@kernel.org>
  25 Reported-by: Jiufei Xue <xuejiufei@gmail.com>
  26 Link: http://lkml.kernel.org/r/dc694ae2-f07f-61e1-7097-7c8411cee12d@gmail.com
  27 Acked-by: Jan Kara <jack@suse.cz>
  28 Signed-off-by: Jens Axboe <axboe@kernel.dk>
  29 Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
  30
  31 ---
  32  fs/fs-writeback.c                |   40 +++++++++++++++++++++++++++++++++++++--
  33  include/linux/backing-dev-defs.h |    1
  34  mm/backing-dev.c                 |    1
  35  3 files changed, 40 insertions(+), 2 deletions(-)
  36
  37 --- a/fs/fs-writeback.c
  38 +++ b/fs/fs-writeback.c
  39 @@ -331,11 +331,22 @@ struct inode_switch_wbs_context {
  40         struct work_struct      work;
  41  };
  42
  43 +static void bdi_down_write_wb_switch_rwsem(struct backing_dev_info *bdi)
  44 +{
  45 +       down_write(&bdi->wb_switch_rwsem);
  46 +}
  47 +
  48 +static void bdi_up_write_wb_switch_rwsem(struct backing_dev_info *bdi)
  49 +{
  50 +       up_write(&bdi->wb_switch_rwsem);
  51 +}
  52 +
  53  static void inode_switch_wbs_work_fn(struct work_struct *work)
  54  {
  55         struct inode_switch_wbs_context *isw =
  56                 container_of(work, struct inode_switch_wbs_context, work);
  57         struct inode *inode = isw->inode;
  58 +       struct backing_dev_info *bdi = inode_to_bdi(inode);
  59         struct address_space *mapping = inode->i_mapping;
  60         struct bdi_writeback *old_wb = inode->i_wb;
  61         struct bdi_writeback *new_wb = isw->new_wb;
  62 @@ -344,6 +355,12 @@ static void inode_switch_wbs_work_fn(str
  63         void **slot;
  64
  65         /*
  66 +        * If @inode switches cgwb membership while sync_inodes_sb() is
  67 +        * being issued, sync_inodes_sb() might miss it.  Synchronize.
  68 +        */
  69 +       down_read(&bdi->wb_switch_rwsem);
  70 +
  71 +       /*
  72          * By the time control reaches here, RCU grace period has passed
  73          * since I_WB_SWITCH assertion and all wb stat update transactions
  74          * between unlocked_inode_to_wb_begin/end() are guaranteed to be
  75 @@ -435,6 +452,8 @@ skip_switch:
  76         spin_unlock(&new_wb->list_lock);
  77         spin_unlock(&old_wb->list_lock);
  78
  79 +       up_read(&bdi->wb_switch_rwsem);
  80 +
  81         if (switched) {
  82                 wb_wakeup(new_wb);
  83                 wb_put(old_wb);
  84 @@ -475,9 +494,18 @@ static void inode_switch_wbs(struct inod
  85         if (inode->i_state & I_WB_SWITCH)
  86                 return;
  87
  88 +       /*
  89 +        * Avoid starting new switches while sync_inodes_sb() is in
  90 +        * progress.  Otherwise, if the down_write protected issue path
  91 +        * blocks heavily, we might end up starting a large number of
  92 +        * switches which will block on the rwsem.
  93 +        */
  94 +       if (!down_read_trylock(&bdi->wb_switch_rwsem))
  95 +               return;
  96 +
  97         isw = kzalloc(sizeof(*isw), GFP_ATOMIC);
  98         if (!isw)
  99 -               return;
 100 +               goto out_unlock;
 101
 102         /* find and pin the new wb */
 103         rcu_read_lock();
 104 @@ -511,12 +539,14 @@ static void inode_switch_wbs(struct inod
 105          * Let's continue after I_WB_SWITCH is guaranteed to be visible.
 106          */
 107         call_rcu(&isw->rcu_head, inode_switch_wbs_rcu_fn);
 108 -       return;
 109 +       goto out_unlock;
 110
 111  out_free:
 112         if (isw->new_wb)
 113                 wb_put(isw->new_wb);
 114         kfree(isw);
 115 +out_unlock:
 116 +       up_read(&bdi->wb_switch_rwsem);
 117  }
 118
 119  /**
 120 @@ -896,6 +926,9 @@ fs_initcall(cgroup_writeback_init);
 121
 122  #else  /* CONFIG_CGROUP_WRITEBACK */
 123
 124 +static void bdi_down_write_wb_switch_rwsem(struct backing_dev_info *bdi) { }
 125 +static void bdi_up_write_wb_switch_rwsem(struct backing_dev_info *bdi) { }
 126 +
 127  static struct bdi_writeback *
 128  locked_inode_to_wb_and_lock_list(struct inode *inode)
 129         __releases(&inode->i_lock)
 130 @@ -2341,8 +2374,11 @@ void sync_inodes_sb(struct super_block *
 131                 return;
 132         WARN_ON(!rwsem_is_locked(&sb->s_umount));
 133
 134 +       /* protect against inode wb switch, see inode_switch_wbs_work_fn() */
 135 +       bdi_down_write_wb_switch_rwsem(bdi);
 136         bdi_split_work_to_wbs(bdi, &work, false);
 137         wb_wait_for_completion(bdi, &done);
 138 +       bdi_up_write_wb_switch_rwsem(bdi);
 139
 140         wait_sb_inodes(sb);
 141  }
 142 --- a/include/linux/backing-dev-defs.h
 143 +++ b/include/linux/backing-dev-defs.h
 144 @@ -157,6 +157,7 @@ struct backing_dev_info {
 145         struct radix_tree_root cgwb_tree; /* radix tree of active cgroup wbs */
 146         struct rb_root cgwb_congested_tree; /* their congested states */
 147         atomic_t usage_cnt; /* counts both cgwbs and cgwb_contested's */
 148 +       struct rw_semaphore wb_switch_rwsem; /* no cgwb switch while syncing */
 149  #else
 150         struct bdi_writeback_congested *wb_congested;
 151  #endif
 152 --- a/mm/backing-dev.c
 153 +++ b/mm/backing-dev.c
 154 @@ -669,6 +669,7 @@ static int cgwb_bdi_init(struct backing_
 155         INIT_RADIX_TREE(&bdi->cgwb_tree, GFP_ATOMIC);
 156         bdi->cgwb_congested_tree = RB_ROOT;
 157         atomic_set(&bdi->usage_cnt, 1);
 158 +       init_rwsem(&bdi->wb_switch_rwsem);
 159
 160         ret = wb_init(&bdi->wb, bdi, 1, GFP_KERNEL);
 161         if (!ret) {