On 2018年04月12日 18:59, Nikolay Borisov wrote:
> 
> 
> On 12.04.2018 13:00, Qu Wenruo wrote:
>> The original first key check is handled in
>> btree_read_extent_buffer_pages(), which is good enough for level check,
>> but to verify first key, we need at least read lock on the extent
>> buffer, or we can hit some race and cause false alert.
>>
>> Fix it by adding new verifcation function: btrfs_verify_first_key(),
>> which will first check lock context of both parent and child extent
>> buffer, and only when we have proper lock hold (write or read lock if
> So if the extent buffer is not in commit root, i.e it's active buffer in
> current transaction (am I right?)

That's right.

> then we need write lock? It's not entirely clearly from that sentence.

Not really write lock. Either read or write (both spinning and blocking)
is OK here.

> 
>> extent buffer is not in commit root) we will verify the first key.
>>
>> Most of the verification happens in ctree.c after function call like
>> read_tree_block(), read_node_slot() and read_block_for_search(), and
>> only call btrfs_verify_first_key() after we acquire read/write lock.
>>
>> Also, since first_key check is not as good as level and could easily
>> trigger false alerts due to lock context, put it under
>> CONFIG_BTRFS_FS_CHECK_INTEGRITY to avoid damage to end user.
> 
> Why is first_key check not as good as level

Several reasons.

1) Level tree check can prevent loop/nodes underflow caused by level
   mismatch, but first_key can't
   One problem we're trying to solve is, when level 1 node points to
   another level 1 tree, we can still try to read next level tree block.
   Which could underflow level.
   first_key check can't really handle it.

2) first_key needs extra lock to read out needed info, while level
   check doesn't (hence easier and harder to cause false alert)
   For a given tree block, its level won't change either for active eb
   (modified in current trans) or committed eb.
   But for first_key, we need proper lock to avoid race.

For short, first key check needs a lot of carefully reviewed check
timing and error handlers, while most problem could be exposed by level
check far before it hits first_key check.

> and why could it trigger
> false alert, given this patch allegedly implements it under proper lock
> context, thus eliminating false positives.

I mean the old and merged first_key check patch.

Thanks,
Qu

> 
>>
>> Signed-off-by: Qu Wenruo <w...@suse.com>
>> ---
>>  fs/btrfs/ctree.c       | 69 ++++++++++++++++++++++++++++++++++++++++++
>>  fs/btrfs/ctree.h       | 58 +++++++++++++++++++++++++++++++++++
>>  fs/btrfs/extent-tree.c |  6 ++++
>>  fs/btrfs/qgroup.c      |  5 +++
>>  fs/btrfs/ref-verify.c  |  6 ++++
>>  fs/btrfs/relocation.c  | 13 ++++++++
>>  6 files changed, 157 insertions(+)
>>
>> diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c
>> index e044b51a2789..57721aead371 100644
>> --- a/fs/btrfs/ctree.c
>> +++ b/fs/btrfs/ctree.c
>> @@ -1649,6 +1649,11 @@ int btrfs_realloc_node(struct btrfs_trans_handle 
>> *trans,
>>  
>>              btrfs_tree_lock(cur);
>>              btrfs_set_lock_blocking(cur);
>> +            if (btrfs_verify_first_key(cur, parent, i)) {
>> +                    err = -EUCLEAN;
>> +                    btrfs_tree_unlock(cur);
>> +                    free_extent_buffer(cur);
>> +            }
>>              err = __btrfs_cow_block(trans, root, cur, parent, i,
>>                                      &cur, search_start,
>>                                      min(16 * blocksize,
>> @@ -1863,6 +1868,12 @@ static noinline int balance_level(struct 
>> btrfs_trans_handle *trans,
>>  
>>              btrfs_tree_lock(child);
>>              btrfs_set_lock_blocking(child);
>> +            if (btrfs_verify_first_key(child, mid, 0)) {
>> +                    ret = -EUCLEAN;
>> +                    btrfs_tree_unlock(child);
>> +                    free_extent_buffer(child);
>> +                    goto enospc;
>> +            }
>>              ret = btrfs_cow_block(trans, root, child, mid, 0, &child);
>>              if (ret) {
>>                      btrfs_tree_unlock(child);
>> @@ -1901,6 +1912,10 @@ static noinline int balance_level(struct 
>> btrfs_trans_handle *trans,
>>      if (left) {
>>              btrfs_tree_lock(left);
>>              btrfs_set_lock_blocking(left);
>> +            if (btrfs_verify_first_key(left, parent, pslot - 1)) {
>> +                    ret = -EUCLEAN;
>> +                    goto enospc;
>> +            }
>>              wret = btrfs_cow_block(trans, root, left,
>>                                     parent, pslot - 1, &left);
>>              if (wret) {
>> @@ -1916,6 +1931,10 @@ static noinline int balance_level(struct 
>> btrfs_trans_handle *trans,
>>      if (right) {
>>              btrfs_tree_lock(right);
>>              btrfs_set_lock_blocking(right);
>> +            if (btrfs_verify_first_key(right, parent, pslot + 1)) {
>> +                    ret = -EUCLEAN;
>> +                    goto enospc;
>> +            }
>>              wret = btrfs_cow_block(trans, root, right,
>>                                     parent, pslot + 1, &right);
>>              if (wret) {
>> @@ -2079,6 +2098,8 @@ static noinline int push_nodes_for_insert(struct 
>> btrfs_trans_handle *trans,
>>  
>>              btrfs_tree_lock(left);
>>              btrfs_set_lock_blocking(left);
>> +            if (btrfs_verify_first_key(left, parent, pslot - 1))
>> +                    goto left_out;
>>  
>>              left_nr = btrfs_header_nritems(left);
>>              if (left_nr >= BTRFS_NODEPTRS_PER_BLOCK(fs_info) - 1) {
>> @@ -2119,6 +2140,7 @@ static noinline int push_nodes_for_insert(struct 
>> btrfs_trans_handle *trans,
>>                      }
>>                      return 0;
>>              }
>> +left_out:
>>              btrfs_tree_unlock(left);
>>              free_extent_buffer(left);
>>      }
>> @@ -2134,6 +2156,8 @@ static noinline int push_nodes_for_insert(struct 
>> btrfs_trans_handle *trans,
>>  
>>              btrfs_tree_lock(right);
>>              btrfs_set_lock_blocking(right);
>> +            if (btrfs_verify_first_key(right, parent, pslot + 1))
>> +                    goto right_out;
>>  
>>              right_nr = btrfs_header_nritems(right);
>>              if (right_nr >= BTRFS_NODEPTRS_PER_BLOCK(fs_info) - 1) {
>> @@ -2174,6 +2198,7 @@ static noinline int push_nodes_for_insert(struct 
>> btrfs_trans_handle *trans,
>>                      }
>>                      return 0;
>>              }
>> +right_out:
>>              btrfs_tree_unlock(right);
>>              free_extent_buffer(right);
>>      }
>> @@ -2868,6 +2893,11 @@ int btrfs_search_slot(struct btrfs_trans_handle 
>> *trans, struct btrfs_root *root,
>>                                      p->locks[level] = BTRFS_READ_LOCK;
>>                              }
>>                              p->nodes[level] = b;
>> +                            if (btrfs_verify_first_key(b, p->nodes[level + 
>> 1],
>> +                                                       slot)) {
>> +                                    ret = -EUCLEAN;
>> +                                    goto done;
>> +                            }
>>                      }
>>              } else {
>>                      p->slots[level] = slot;
>> @@ -2981,6 +3011,12 @@ int btrfs_search_old_slot(struct btrfs_root *root, 
>> const struct btrfs_key *key,
>>                              goto done;
>>                      }
>>  
>> +                    /*
>> +                     * NOTE: both parent and child go through
>> +                     * tree_mod_log_rewind(), first key check is no
>> +                     * longer consistent. So no btrfs_verify_first_key()
>> +                     * for this read_block_for_search().
>> +                     */
>>                      err = read_block_for_search(root, p, &b, level,
>>                                                  slot, key);
>>                      if (err == -EAGAIN)
>> @@ -3753,6 +3789,8 @@ static int push_leaf_right(struct btrfs_trans_handle 
>> *trans, struct btrfs_root
>>  
>>      btrfs_tree_lock(right);
>>      btrfs_set_lock_blocking(right);
>> +    if (btrfs_verify_first_key(right, upper, slot + 1))
>> +            goto out_unlock;
>>  
>>      free_space = btrfs_leaf_free_space(fs_info, right);
>>      if (free_space < data_size)
>> @@ -3987,6 +4025,10 @@ static int push_leaf_left(struct btrfs_trans_handle 
>> *trans, struct btrfs_root
>>  
>>      btrfs_tree_lock(left);
>>      btrfs_set_lock_blocking(left);
>> +    if (btrfs_verify_first_key(left, path->nodes[1], slot - 1)) {
>> +            ret = 1;
>> +            goto out;
>> +    }
>>  
>>      free_space = btrfs_leaf_free_space(fs_info, left);
>>      if (free_space < data_size) {
>> @@ -5205,6 +5247,12 @@ int btrfs_search_forward(struct btrfs_root *root, 
>> struct btrfs_key *min_key,
>>              }
>>  
>>              btrfs_tree_read_lock(cur);
>> +            if (btrfs_verify_first_key(cur, path->nodes[level], slot)) {
>> +                    btrfs_tree_read_unlock(cur);
>> +                    free_extent_buffer(cur);
>> +                    ret = -EUCLEAN;
>> +                    goto out;
>> +            }
>>  
>>              path->locks[level - 1] = BTRFS_READ_LOCK;
>>              path->nodes[level - 1] = cur;
>> @@ -5232,6 +5280,11 @@ static int tree_move_down(struct btrfs_fs_info 
>> *fs_info,
>>      if (IS_ERR(eb))
>>              return PTR_ERR(eb);
>>  
>> +    if (btrfs_verify_first_key(eb, path->nodes[*level],
>> +                               path->slots[*level])) {
>> +            free_extent_buffer(eb);
>> +            return -EUCLEAN;
>> +    }
>>      path->nodes[*level - 1] = eb;
>>      path->slots[*level - 1] = 0;
>>      (*level)--;
>> @@ -5786,6 +5839,14 @@ int btrfs_next_old_leaf(struct btrfs_root *root, 
>> struct btrfs_path *path,
>>                                                        BTRFS_READ_LOCK);
>>                      }
>>                      next_rw_lock = BTRFS_READ_LOCK;
>> +                    if (btrfs_verify_first_key(next, path->nodes[level],
>> +                                               slot)) {
>> +                            ret = -EUCLEAN;
>> +                            btrfs_tree_read_unlock(next);
>> +                            free_extent_buffer(next);
>> +                            btrfs_release_path(path);
>> +                            goto done;
>> +                    }
>>              }
>>              break;
>>      }
>> @@ -5823,6 +5884,14 @@ int btrfs_next_old_leaf(struct btrfs_root *root, 
>> struct btrfs_path *path,
>>                                                        BTRFS_READ_LOCK);
>>                      }
>>                      next_rw_lock = BTRFS_READ_LOCK;
>> +                    if (btrfs_verify_first_key(next, path->nodes[level],
>> +                                               0)) {
>> +                            ret = -EUCLEAN;
>> +                            btrfs_tree_read_unlock(next);
>> +                            free_extent_buffer(next);
>> +                            btrfs_release_path(path);
>> +                            goto done;
>> +                    }
>>              }
>>      }
>>      ret = 0;
>> diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
>> index 0eb55825862a..033d8ae027c3 100644
>> --- a/fs/btrfs/ctree.h
>> +++ b/fs/btrfs/ctree.h
>> @@ -44,6 +44,7 @@
>>  #include "extent_io.h"
>>  #include "extent_map.h"
>>  #include "async-thread.h"
>> +#include "print-tree.h"
>>  
>>  struct btrfs_trans_handle;
>>  struct btrfs_transaction;
>> @@ -3752,4 +3753,61 @@ static inline int btrfs_is_testing(struct 
>> btrfs_fs_info *fs_info)
>>  #endif
>>      return 0;
>>  }
>> +
>> +/*
>> + * Verify the first key of an extent buffer matches with its parent pointer.
>> + *
>> + * For tree blocks in current transaction, caller should hold at least read 
>> lock
>> + * for both @eb and @parent to avoid race.
>> + * While for tree blocks in commit root, no such requirement.
>> + */
>> +static inline int btrfs_verify_first_key(struct extent_buffer *eb,
>> +                                     struct extent_buffer *parent, int slot)
>> +{
>> +#ifdef CONFIG_BTRFS_FS_CHECK_INTEGRITY
>> +    struct btrfs_key parent_key;
>> +    struct btrfs_key found_key;
>> +    struct btrfs_fs_info *fs_info;
>> +    u64 transid = 0;
>> +    int ret;
>> +
>> +    if (!parent || !eb)
>> +            return 0;
>> +    fs_info = parent->fs_info;
>> +    transid = fs_info->last_trans_committed;
>> +    /*
>> +     * For first_key check, we need to ensure we have proper lock hold,
>> +     * or the content may change and cause false alert.
>> +     */
>> +    if (btrfs_header_generation(eb) > transid &&
>> +        !atomic_read(&eb->read_locks) && !atomic_read(&eb->write_locks)) {
>> +            WARN_ON(1);
>> +            return 0;
>> +    }
>> +    if (btrfs_header_generation(parent) > transid &&
>> +        !atomic_read(&parent->read_locks) &&
>> +        !atomic_read(&parent->write_locks)) {
>> +            WARN_ON(1);
>> +            return 0;
>> +    }
>> +    btrfs_node_key_to_cpu(parent, &parent_key, slot);
>> +    if (btrfs_header_level(eb))
>> +            btrfs_node_key_to_cpu(eb, &found_key, 0);
>> +    else
>> +            btrfs_item_key_to_cpu(eb, &found_key, 0);
>> +    ret =  btrfs_comp_cpu_keys(&found_key, &parent_key);
>> +    if (ret) {
>> +            WARN_ON(ret);
>> +            btrfs_err(fs_info,
>> +"tree first key mismatch detected, bytenr=%llu key expected=(%llu, %u, 
>> %llu) has=(%llu, %u, %llu)",
>> +                      eb->start, parent_key.objectid, parent_key.type,
>> +                      parent_key.offset, found_key.objectid,
>> +                      found_key.type, found_key.offset);
>> +    }
>> +    return ret;
>> +#else
>> +    return 0;
>> +#endif /* CONFIG_BTRFS_FS_CHECK_INTEGRITY */
>> +}
>> +
>>  #endif
>> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
>> index f080834e4dd0..17bd22a54281 100644
>> --- a/fs/btrfs/extent-tree.c
>> +++ b/fs/btrfs/extent-tree.c
>> @@ -8807,6 +8807,12 @@ static noinline int do_walk_down(struct 
>> btrfs_trans_handle *trans,
>>              }
>>              btrfs_tree_lock(next);
>>              btrfs_set_lock_blocking(next);
>> +            if (btrfs_verify_first_key(next, path->nodes[level],
>> +                                       path->slots[level])) {
>> +                    btrfs_tree_unlock(next);
>> +                    free_extent_buffer(next);
>> +                    goto out_unlock;
>> +            }
>>      }
>>  
>>      level--;
>> diff --git a/fs/btrfs/qgroup.c b/fs/btrfs/qgroup.c
>> index c7f129356966..4c53954297d8 100644
>> --- a/fs/btrfs/qgroup.c
>> +++ b/fs/btrfs/qgroup.c
>> @@ -1744,6 +1744,11 @@ int btrfs_qgroup_trace_subtree(struct 
>> btrfs_trans_handle *trans,
>>  
>>                      btrfs_tree_read_lock(eb);
>>                      btrfs_set_lock_blocking_rw(eb, BTRFS_READ_LOCK);
>> +                    if (btrfs_verify_first_key(eb, path->nodes[level + 1],
>> +                                               parent_slot)) {
>> +                            ret = -EUCLEAN;
>> +                            goto out;
>> +                    }
>>                      path->locks[level] = BTRFS_READ_LOCK_BLOCKING;
>>  
>>                      ret = btrfs_qgroup_trace_extent(trans, fs_info,
>> diff --git a/fs/btrfs/ref-verify.c b/fs/btrfs/ref-verify.c
>> index 6d856eabf34a..b996ca1fd9dc 100644
>> --- a/fs/btrfs/ref-verify.c
>> +++ b/fs/btrfs/ref-verify.c
>> @@ -593,6 +593,12 @@ static int walk_down_tree(struct btrfs_root *root, 
>> struct btrfs_path *path,
>>                      }
>>                      btrfs_tree_read_lock(eb);
>>                      btrfs_set_lock_blocking_rw(eb, BTRFS_READ_LOCK);
>> +                    if (btrfs_verify_first_key(eb, path->nodes[level],
>> +                        path->slots[level])) {
>> +                            btrfs_tree_read_unlock_blocking(eb);
>> +                            free_extent_buffer(eb);
>> +                            return -EUCLEAN;
>> +                    }
>>                      path->nodes[level-1] = eb;
>>                      path->slots[level-1] = 0;
>>                      path->locks[level-1] = BTRFS_READ_LOCK_BLOCKING;
>> diff --git a/fs/btrfs/relocation.c b/fs/btrfs/relocation.c
>> index 74db4694d776..74a088fbdac4 100644
>> --- a/fs/btrfs/relocation.c
>> +++ b/fs/btrfs/relocation.c
>> @@ -1888,6 +1888,13 @@ int replace_path(struct btrfs_trans_handle *trans,
>>                              break;
>>                      }
>>                      btrfs_tree_lock(eb);
>> +                    if (btrfs_verify_first_key(eb, parent, slot)) {
>> +                            btrfs_tree_unlock(eb);
>> +                            free_extent_buffer(eb);
>> +                            ret = -EUCLEAN;
>> +                            break;
>> +                    }
>> +
>>                      if (cow) {
>>                              ret = btrfs_cow_block(trans, dest, eb, parent,
>>                                                    slot, &eb);
>> @@ -2793,6 +2800,12 @@ static int do_relocation(struct btrfs_trans_handle 
>> *trans,
>>              }
>>              btrfs_tree_lock(eb);
>>              btrfs_set_lock_blocking(eb);
>> +            if (btrfs_verify_first_key(eb, upper->eb, slot)) {
>> +                    btrfs_tree_unlock(eb);
>> +                    free_extent_buffer(eb);
>> +                    err = -EUCLEAN;
>> +                    goto next;
>> +            }
>>  
>>              if (!node->eb) {
>>                      ret = btrfs_cow_block(trans, root, eb, upper->eb,
>>
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