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Filipe Manana authored
While iterating and copying extents from the source device, the device replace code keeps adjusting a left cursor that is used to make sure that once we finish processing a device extent, any future writes to extents from the corresponding block group will get into both the source and target devices. This left cursor is also used for resuming the device replace operation at mount time. However using this left cursor to decide whether writes go into both devices or only the source device is not enough to guarantee we don't miss copying extents into the target device. There are two cases where the current approach fails. The first one is related to when there are holes in the device and they get allocated for new block groups while the device replace operation is iterating the device extents (more on this explained below). The second one is that when that loop over the device extents finishes, we start dellaloc, wait for all ordered extents and then commit the current transaction, we might have got new block groups allocated that are now using a device extent that has an offset greater then or equals to the value of the left cursor, in which case writes to extents belonging to these new block groups will get issued only to the source device. For the first case where the current approach of using a left cursor fails, consider the source device currently has the following layout: [ extent bg A ] [ hole, unallocated space ] [extent bg B ] 3Gb 4Gb 5Gb While we are iterating the device extents from the source device using the commit root of the device tree, the following happens: CPU 1 CPU 2 <we are at transaction N> scrub_enumerate_chunks() --> searches the device tree for extents belonging to the source device using the device tree's commit root --> 1st iteration finds extent belonging to block group A --> sets block group A to RO mode (btrfs_inc_block_group_ro) --> sets cursor left to found_key.offset which is 3Gb --> scrub_chunk() starts copies all allocated extents from block group's A stripe at source device into target device btrfs_alloc_chunk() --> allocates device extent in the range [4Gb, 5Gb[ from the source device for a new block group C extent allocated from block group C for a direct IO, buffered write or btree node/leaf extent is written to, perhaps in response to a writepages() call from the VM or directly through direct IO the write is made only against the source device and not against the target device because the extent's offset is in the interval [4Gb, 5Gb[ which is larger then the value of cursor_left (3Gb) --> scrub_chunks() finishes --> updates left cursor from 3Gb to 4Gb --> btrfs_dec_block_group_ro() sets block group A back to RW mode <we are still at transaction N> --> 2nd iteration finds extent belonging to block group B - it did not find the new extent in the range [4Gb, 5Gb[ for block group C because we are using the device tree's commit root or even because the block group's items are not all yet inserted in the respective btrees, that is, the block group is still attached to some transaction handle's new_bgs list and btrfs_create_pending_block_groups() was not called yet against that transaction handle, so the device extent items were not yet inserted into the devices tree <we are still at transaction N> --> so we end not copying anything from the newly allocated device extent from the source device to the target device So fix this by making __btrfs_map_block() always redirect writes to the target device as well, independently of the left cursor's value. With this change the left cursor is now used only for the purpose of tracking progress and allow a mount operation to resume a device replace. Signed-off-by:
Filipe Manana <fdmanana@suse.com> Reviewed-by:
Josef Bacik <jbacik@fb.com>
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