diff options
Diffstat (limited to 'fs/ext4/extents_status.c')
| -rw-r--r-- | fs/ext4/extents_status.c | 35 |
1 files changed, 33 insertions, 2 deletions
diff --git a/fs/ext4/extents_status.c b/fs/ext4/extents_status.c index d1401d4a5513..31dc0496f8d0 100644 --- a/fs/ext4/extents_status.c +++ b/fs/ext4/extents_status.c @@ -120,9 +120,40 @@ * memory. Hence, we will reclaim written/unwritten/hole extents from * the tree under a heavy memory pressure. * + * ========================================================================== + * 3. Assurance of Ext4 extent status tree consistency + * + * When mapping blocks, Ext4 queries the extent status tree first and should + * always trusts that the extent status tree is consistent and up to date. + * Therefore, it is important to adheres to the following rules when createing, + * modifying and removing extents. + * + * 1. Besides fastcommit replay, when Ext4 creates or queries block mappings, + * the extent information should always be processed through the extent + * status tree instead of being organized manually through the on-disk + * extent tree. + * + * 2. When updating the extent tree, Ext4 should acquire the i_data_sem + * exclusively and update the extent status tree atomically. If the extents + * to be modified are large enough to exceed the range that a single + * i_data_sem can process (as ext4_datasem_ensure_credits() may drop + * i_data_sem to restart a transaction), it must (e.g. as ext4_punch_hole() + * does): + * + * a) Hold the i_rwsem and invalidate_lock exclusively. This ensures + * exclusion against page faults, as well as reads and writes that may + * concurrently modify the extent status tree. + * b) Evict all page cache in the affected range and recommend rebuilding + * or dropping the extent status tree after modifying the on-disk + * extent tree. This ensures exclusion against concurrent writebacks + * that do not hold those locks but only holds a folio lock. + * + * 3. Based on the rules above, when querying block mappings, Ext4 should at + * least hold the i_rwsem or invalidate_lock or folio lock(s) for the + * specified querying range. * * ========================================================================== - * 3. Performance analysis + * 4. Performance analysis * * -- overhead * 1. There is a cache extent for write access, so if writes are @@ -134,7 +165,7 @@ * * * ========================================================================== - * 4. TODO list + * 5. TODO list * * -- Refactor delayed space reservation * |