Thank you, Filipe. Now it is more clear. Fortunately, in my kernel 3.18 I do not have do_chunk_alloc() calling btrfs_create_pending_block_groups(), so I cannot hit this deadlock. But can hit the issue that this call is meant to fix.
Thanks, Alex. On Sun, Dec 13, 2015 at 5:45 PM, Filipe Manana <fdman...@kernel.org> wrote: > 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
