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? 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
