On 11.04.2018 10:09, Omar Sandoval wrote:
> On Wed, Apr 11, 2018 at 10:04:49AM +0300, Nikolay Borisov wrote:
>> When the delayed refs for a head are all run, eventually
>> cleanup_ref_head is called which (in case of deletion) obtains a
>> reference for the relevant btrfs_space_info struct by querying the bg
>> for the range. This is problematic because when the last extent of a
>> bg is deleted a race window emerges between removal of that bg and the
>> subsequent invocation of cleanup_ref_head. This can result in cache being null
>> and either a null pointer dereference or assertion failure.
>>
>> task: ffff8d04d31ed080 task.stack: ffff9e5dc10cc000
>> RIP: 0010:assfail.constprop.78+0x18/0x1a [btrfs]
>> RSP: 0018:ffff9e5dc10cfbe8 EFLAGS: 00010292
>> RAX: 0000000000000044 RBX: 0000000000000000 RCX: 0000000000000000
>> RDX: ffff8d04ffc1f868 RSI: ffff8d04ffc178c8 RDI: ffff8d04ffc178c8
>> RBP: ffff8d04d29e5ea0 R08: 00000000000001f0 R09: 0000000000000001
>> R10: ffff9e5dc0507d58 R11: 0000000000000001 R12: ffff8d04d29e5ea0
>> R13: ffff8d04d29e5f08 R14: ffff8d04efe29b40 R15: ffff8d04efe203e0
>> FS: 00007fbf58ead500(0000) GS:ffff8d04ffc00000(0000) knlGS:0000000000000000
>> CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>> CR2: 00007fe6c6975648 CR3: 0000000013b2a000 CR4: 00000000000006f0
>> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
>> Call Trace:
>> __btrfs_run_delayed_refs+0x10e7/0x12c0 [btrfs]
>> btrfs_run_delayed_refs+0x68/0x250 [btrfs]
>> btrfs_should_end_transaction+0x42/0x60 [btrfs]
>> btrfs_truncate_inode_items+0xaac/0xfc0 [btrfs]
>> btrfs_evict_inode+0x4c6/0x5c0 [btrfs]
>> evict+0xc6/0x190
>> do_unlinkat+0x19c/0x300
>> do_syscall_64+0x74/0x140
>> entry_SYSCALL_64_after_hwframe+0x3d/0xa2
>> RIP: 0033:0x7fbf589c57a7
>>
>> To fix this, introduce a new flag "is_system" to head_ref structs,
>> which is populated at insertion time. This allows to decouple the
>> querying for the spaceinfo from querying the possibly deleted bg.
>>
>> Fixes: d7eae3403f46 ("Btrfs: rework delayed ref total_bytes_pinned accounting")
>> Suggested-by: Omar Sandoval <osandov@xxxxxxxxxxx>
>> Signed-off-by: Nikolay Borisov <nborisov@xxxxxxxx>
>> ---
>> fs/btrfs/delayed-ref.c | 19 ++++++++++++++-----
>> fs/btrfs/delayed-ref.h | 1 +
>> fs/btrfs/extent-tree.c | 20 +++++++++++++++-----
>> 3 files changed, 30 insertions(+), 10 deletions(-)
>>
>> diff --git a/fs/btrfs/delayed-ref.c b/fs/btrfs/delayed-ref.c
>> index 5ac2d7909782..20fa7b4c132d 100644
>> --- a/fs/btrfs/delayed-ref.c
>> +++ b/fs/btrfs/delayed-ref.c
>> @@ -550,8 +550,10 @@ add_delayed_ref_head(struct btrfs_fs_info *fs_info,
>> struct btrfs_delayed_ref_head *head_ref,
>> struct btrfs_qgroup_extent_record *qrecord,
>> u64 bytenr, u64 num_bytes, u64 ref_root, u64 reserved,
>> - int action, int is_data, int *qrecord_inserted_ret,
>> + int action, int is_data, int is_system,
>> + int *qrecord_inserted_ret,
>> int *old_ref_mod, int *new_ref_mod)
>> +
>> {
>> struct btrfs_delayed_ref_head *existing;
>> struct btrfs_delayed_ref_root *delayed_refs;
>> @@ -595,6 +597,7 @@ add_delayed_ref_head(struct btrfs_fs_info *fs_info,
>> head_ref->ref_mod = count_mod;
>> head_ref->must_insert_reserved = must_insert_reserved;
>> head_ref->is_data = is_data;
>> + head_ref->is_system = is_system;
>> head_ref->ref_tree = RB_ROOT;
>> INIT_LIST_HEAD(&head_ref->ref_add_list);
>> RB_CLEAR_NODE(&head_ref->href_node);
>> @@ -782,6 +785,7 @@ int btrfs_add_delayed_tree_ref(struct btrfs_fs_info *fs_info,
>> struct btrfs_delayed_ref_root *delayed_refs;
>> struct btrfs_qgroup_extent_record *record = NULL;
>> int qrecord_inserted;
>> + int is_system = ref_root == BTRFS_CHUNK_TREE_OBJECTID;
>>
>> BUG_ON(extent_op && extent_op->is_data);
>> ref = kmem_cache_alloc(btrfs_delayed_tree_ref_cachep, GFP_NOFS);
>> @@ -810,8 +814,8 @@ int btrfs_add_delayed_tree_ref(struct btrfs_fs_info *fs_info,
>> */
>> head_ref = add_delayed_ref_head(fs_info, trans, head_ref, record,
>> bytenr, num_bytes, 0, 0, action, 0,
>> - &qrecord_inserted, old_ref_mod,
>> - new_ref_mod);
>> + is_system, &qrecord_inserted,
>> + old_ref_mod, new_ref_mod);
>>
>> add_delayed_tree_ref(fs_info, trans, head_ref, &ref->node, bytenr,
>> num_bytes, parent, ref_root, level, action);
>> @@ -878,7 +882,7 @@ int btrfs_add_delayed_data_ref(struct btrfs_fs_info *fs_info,
>> */
>> head_ref = add_delayed_ref_head(fs_info, trans, head_ref, record,
>> bytenr, num_bytes, ref_root, reserved,
>> - action, 1, &qrecord_inserted,
>> + action, 1, 0, &qrecord_inserted,
>> old_ref_mod, new_ref_mod);
>>
>> add_delayed_data_ref(fs_info, trans, head_ref, &ref->node, bytenr,
>> @@ -908,9 +912,14 @@ int btrfs_add_delayed_extent_op(struct btrfs_fs_info *fs_info,
>> delayed_refs = &trans->transaction->delayed_refs;
>> spin_lock(&delayed_refs->lock);
>>
>> + /*
>> + * extent_ops just modify the flags of an extent and they don't result
>> + * in ref count changes, hence it's safe to pass false/0 for is_system
>> + * argument
>> + */
>> add_delayed_ref_head(fs_info, trans, head_ref, NULL, bytenr,
>> num_bytes, 0, 0, BTRFS_UPDATE_DELAYED_HEAD,
>> - extent_op->is_data, NULL, NULL, NULL);
>> + extent_op->is_data, 0, NULL, NULL, NULL);
>>
>> spin_unlock(&delayed_refs->lock);
>> return 0;
>> diff --git a/fs/btrfs/delayed-ref.h b/fs/btrfs/delayed-ref.h
>> index cdf4a94ce5c1..7e7bf4e66d48 100644
>> --- a/fs/btrfs/delayed-ref.h
>> +++ b/fs/btrfs/delayed-ref.h
>> @@ -139,6 +139,7 @@ struct btrfs_delayed_ref_head {
>> */
>> unsigned int must_insert_reserved:1;
>> unsigned int is_data:1;
>> + unsigned int is_system:1;
>> unsigned int processing:1;
>> };
>>
>> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
>> index 9a05e5b5089f..ba3468a84f57 100644
>> --- a/fs/btrfs/extent-tree.c
>> +++ b/fs/btrfs/extent-tree.c
>> @@ -2616,13 +2616,23 @@ static int cleanup_ref_head(struct btrfs_trans_handle *trans,
>> trace_run_delayed_ref_head(fs_info, head, 0);
>>
>> if (head->total_ref_mod < 0) {
>> - struct btrfs_block_group_cache *cache;
>> + struct btrfs_space_info *space_info;
>>
>> - cache = btrfs_lookup_block_group(fs_info, head->bytenr);
>> - ASSERT(cache);
>> - percpu_counter_add(&cache->space_info->total_bytes_pinned,
>> + if (head->is_data) {
>> + space_info = __find_space_info(fs_info,
>> + BTRFS_BLOCK_GROUP_DATA);
>> + } else {
>> + if (head->is_system) {
>> + space_info = __find_space_info(fs_info,
>> + BTRFS_BLOCK_GROUP_SYSTEM);
>> + } else {
>> + space_info = __find_space_info(fs_info,
>> + BTRFS_BLOCK_GROUP_METADATA);
>> + }
>
> Could you collapse the nested if/else into a big if (head->is_data) else
> if (head->is_system) else?
>
> Were you able to reproduce this on demand?
I wasn't. The problem is that even if the last extent in a bg is deleted
but the delayed refs are run from the helper thread and not directly
from truncate_inode_items it seems that this is not triggerable. And I
couldn't figure out why delayed refs weren't being run straight from
that function (That code is a mess btw butI have some pending cleanups
in that area).
>
> Besides that,
>
> Reviewed-by: Omar Sandoval <osandov@xxxxxx>
>
> Thanks for fixing this!
>
>> + }
>> + ASSERT(space_info);
>> + percpu_counter_add(&space_info->total_bytes_pinned,
>> -head->num_bytes);
>> - btrfs_put_block_group(cache);
>>
>> if (head->is_data) {
>> spin_lock(&delayed_refs->lock);
>> --
>> 2.7.4
>>
>
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