On Sun, Dec 13, 2015 at 10:29 AM, Alex Lyakas <a...@zadarastorage.com> wrote: > Hi Filipe Manana, > > Can't the call to btrfs_create_pending_block_groups() cause a > deadlock, like in > http://www.spinics.net/lists/linux-btrfs/msg48744.html? Because this > call updates the device tree, and we may be calling do_chunk_alloc() > from find_free_extent() when holding a lock on the device tree root > (because we want to COW a block of the device tree). > > My understanding from Josef's chunk allocator rework > (http://www.spinics.net/lists/linux-btrfs/msg25722.html) was that now > when allocating a new chunk we do not immediately update the > device/chunk tree. We keep the new chunk in "pending_chunks" and in > "new_bgs" on a transaction handle, and we actually update the > chunk/device tree only when we are done with a particular transaction > handle. This way we avoid that sort of deadlocks. > > But this patch breaks this rule, as it may make us update the > device/chunk tree in the context of chunk allocation, which is the > scenario that the rework was meant to avoid. > > Can you please point me at what I am missing?
https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=d9a0540a79f87456907f2ce031f058cf745c5bff > > Thanks, > Alex. > > > On Wed, Jul 22, 2015 at 1:53 AM, Omar Sandoval <osan...@fb.com> wrote: >> On Mon, Jul 20, 2015 at 02:56:20PM +0100, fdman...@kernel.org wrote: >>> From: Filipe Manana <fdman...@suse.com> >>> >>> Omar reported that after commit 4fbcdf669454 ("Btrfs: fix -ENOSPC when >>> finishing block group creation"), introduced in 4.2-rc1, the following >>> test was failing due to exhaustion of the system array in the superblock: >>> >>> #!/bin/bash >>> >>> truncate -s 100T big.img >>> mkfs.btrfs big.img >>> mount -o loop big.img /mnt/loop >>> >>> num=5 >>> sz=10T >>> for ((i = 0; i < $num; i++)); do >>> echo fallocate $i $sz >>> fallocate -l $sz /mnt/loop/testfile$i >>> done >>> btrfs filesystem sync /mnt/loop >>> >>> for ((i = 0; i < $num; i++)); do >>> echo rm $i >>> rm /mnt/loop/testfile$i >>> btrfs filesystem sync /mnt/loop >>> done >>> umount /mnt/loop >>> >>> This made btrfs_add_system_chunk() fail with -EFBIG due to excessive >>> allocation of system block groups. This happened because the test creates >>> a large number of data block groups per transaction and when committing >>> the transaction we start the writeout of the block group caches for all >>> the new new (dirty) block groups, which results in pre-allocating space >>> for each block group's free space cache using the same transaction handle. >>> That in turn often leads to creation of more block groups, and all get >>> attached to the new_bgs list of the same transaction handle to the point >>> of getting a list with over 1500 elements, and creation of new block groups >>> leads to the need of reserving space in the chunk block reserve and often >>> creating a new system block group too. >>> >>> So that made us quickly exhaust the chunk block reserve/system space info, >>> because as of the commit mentioned before, we do reserve space for each >>> new block group in the chunk block reserve, unlike before where we would >>> not and would at most allocate one new system block group and therefore >>> would only ensure that there was enough space in the system space info to >>> allocate 1 new block group even if we ended up allocating thousands of >>> new block groups using the same transaction handle. That worked most of >>> the time because the computed required space at check_system_chunk() is >>> very pessimistic (assumes a chunk tree height of BTRFS_MAX_LEVEL/8 and >>> that all nodes/leafs in a path will be COWed and split) and since the >>> updates to the chunk tree all happen at btrfs_create_pending_block_groups >>> it is unlikely that a path needs to be COWed more than once (unless >>> writepages() for the btree inode is called by mm in between) and that >>> compensated for the need of creating any new nodes/leads in the chunk >>> tree. >>> >>> So fix this by ensuring we don't accumulate a too large list of new block >>> groups in a transaction's handles new_bgs list, inserting/updating the >>> chunk tree for all accumulated new block groups and releasing the unused >>> space from the chunk block reserve whenever the list becomes sufficiently >>> large. This is a generic solution even though the problem currently can >>> only happen when starting the writeout of the free space caches for all >>> dirty block groups (btrfs_start_dirty_block_groups()). >>> >>> Reported-by: Omar Sandoval <osan...@fb.com> >>> Signed-off-by: Filipe Manana <fdman...@suse.com> >> >> Thanks a lot for taking a look. >> >> Tested-by: Omar Sandoval <osan...@fb.com> >> >>> --- >>> fs/btrfs/extent-tree.c | 18 ++++++++++++++++++ >>> 1 file changed, 18 insertions(+) >>> >>> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c >>> index 171312d..07204bf 100644 >>> --- a/fs/btrfs/extent-tree.c >>> +++ b/fs/btrfs/extent-tree.c >>> @@ -4227,6 +4227,24 @@ out: >>> space_info->chunk_alloc = 0; >>> spin_unlock(&space_info->lock); >>> mutex_unlock(&fs_info->chunk_mutex); >>> + /* >>> + * When we allocate a new chunk we reserve space in the chunk block >>> + * reserve to make sure we can COW nodes/leafs in the chunk tree or >>> + * add new nodes/leafs to it if we end up needing to do it when >>> + * inserting the chunk item and updating device items as part of the >>> + * second phase of chunk allocation, performed by >>> + * btrfs_finish_chunk_alloc(). So make sure we don't accumulate a >>> + * large number of new block groups to create in our transaction >>> + * handle's new_bgs list to avoid exhausting the chunk block reserve >>> + * in extreme cases - like having a single transaction create many new >>> + * block groups when starting to write out the free space caches of >>> all >>> + * the block groups that were made dirty during the lifetime of the >>> + * transaction. >>> + */ >>> + if (trans->chunk_bytes_reserved >= (2 * 1024 * 1024ull)) { >>> + btrfs_create_pending_block_groups(trans, trans->root); >>> + btrfs_trans_release_chunk_metadata(trans); >>> + } >>> return ret; >>> } >>> >>> -- >>> 2.1.3 >>> >>> -- >>> To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in >>> the body of a message to majord...@vger.kernel.org >>> More majordomo info at http://vger.kernel.org/majordomo-info.html >> >> -- >> Omar >> -- >> To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in >> the body of a message to majord...@vger.kernel.org >> More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
