2013/12/14 Filipe David Manana <fdmanana@xxxxxxxxx>:
> On Sat, Dec 14, 2013 at 3:13 PM, Filipe David Manana <fdmanana@xxxxxxxxx> wrote:
>> On Sat, Dec 14, 2013 at 3:08 PM, Shilong Wang <wangshilong1991@xxxxxxxxx> wrote:
>>> 2013/12/14 Filipe David Manana <fdmanana@xxxxxxxxx>:
>>>> On Sat, Dec 14, 2013 at 2:56 PM, Shilong Wang <wangshilong1991@xxxxxxxxx> wrote:
>>>>> Hello Filipe,
>>>>>
>>>>> 2013/12/14 Filipe David Borba Manana <fdmanana@xxxxxxxxx>:
>>>>>> Wang Shilong got into a case where during inode eviction we were
>>>>>> removing an extent map while it was pinned. This triggered a warning
>>>>>> in remove_extent_mapping() because the extent map had the pinned
>>>>>> flag set:
>>>>>>
>>>>>> [ 1209.102076] [<ffffffffa04721b9>] remove_extent_mapping+0x69/0x70 [btrfs]
>>>>>> [ 1209.102084] [<ffffffffa0466b06>] btrfs_evict_inode+0x96/0x4d0 [btrfs]
>>>>>> [ 1209.102089] [<ffffffff81073010>] ? wake_atomic_t_function+0x40/0x40
>>>>>> [ 1209.102092] [<ffffffff8118ab2e>] evict+0x9e/0x190
>>>>>> [ 1209.102094] [<ffffffff8118b313>] iput+0xf3/0x180
>>>>>> [ 1209.102101] [<ffffffffa0461fd1>] btrfs_run_delayed_iputs+0xb1/0xd0 [btrfs]
>>>>>> [ 1209.102107] [<ffffffffa045d358>] __btrfs_end_transaction+0x268/0x350 [btrfs]
>>>>>>
>>>>>> Therefore wait for any pending ordered extents, if any, which will
>>>>>> trigger calls to unpin_extent_cache(), before removing the extent maps.
>>>>>>
>>>>>> Wang's solution of simply clearing the pinned bit wasn't enough, as after
>>>>>> unpin_extent_cache() will be called and trigger another WARN_ON() because
>>>>>> the lookup for the extent map returned NULL.
>>>>>
>>>>> Why not in evict_inode_truncate_pages() move remove_extent_mapping() after
>>>>> clear_extent_bit()?
>>>>
>>>> So, if the pinned bit is set, it means some task will clear it later,
>>>> via unpin_extent_cache(). And if you look at that function, it has
>>>> this:
>>>>
>>>> write_lock(&tree->lock);
>>>> em = lookup_extent_mapping(tree, start, len);
>>>>
>>>> WARN_ON(!em || em->start != start);
>>>>
>>>> And remove_extent_mapping() will remove the em from the rbtree,
>>>> regardless of its reference count value, therefore triggering that
>>>> warning above.
>>>
>>> Here i mean, in evict_inode_truncate_pages()
>>> We change it to:
>>>
>>> Step1: unpin_extent_cache()
>>> Step2: remove it from extent_mapping
>>>
>>> Dose this cause any problems? i am a little confused, correct me if i
>>> am wrong some places^_^.
>>
>> It can still lead to the same WARN_ON I think. So when calling
>> unpin_extent_cache(), it can merge the em with its left neighbor,
>> therefore changing its ->start value. So later, if other task (the one
>> which set the pinned flag) calls remove_extent_mapping(), it will get
>> an em with a different ->start (because of the merge), therefore
>> triggering that WARN_ON().
>
> Or because it is not found the second time.
>
> On the other hand, you didn't get such WARN_ON triggered, right?
During my test, i did not hit another WARN_ON in fact.
>
> So maybe just clearing the pinned bit is ok. So btrfs_invalidatepage,
> if it finds an ordered extent, it sets the BTRFS_ORDERED_TRUNCATED
> flag on it, and then it might call btrfs_finish_ordered_io() against
> it, which not always unpins the extent when it has the truncated flag
> set. So this might well be what you ran into.
Let's dig more.
I take a deep look in btrfs_finish_ordered_io(), it won't unset PINNED flag
For an NOCOW extent.
2579 if (test_bit(BTRFS_ORDERED_NOCOW,
&ordered_extent->flags)) {
2580 BUG_ON(!list_empty(&ordered_extent->list)); /*
Logic error */
2581 btrfs_ordered_update_i_size(inode, 0, ordered_extent);
2582 if (nolock)
2583 trans = btrfs_join_transaction_nolock(root);
2584 else
2585 trans = btrfs_join_transaction(root);
2586 if (IS_ERR(trans)) {
2587 ret = PTR_ERR(trans);
2588 trans = NULL;
2589 goto out;
2590 }
2591 trans->block_rsv = &root->fs_info->delalloc_block_rsv;
2592 ret = btrfs_update_inode_fallback(trans, root, inode);
2593 if (ret) /* -ENOMEM or corruption */
2594 btrfs_abort_transaction(trans, root, ret);
2595 goto out;
------------------------->here we goto out directly,
unpin_extent_cache() won't be called.
2596 }
I don't know why we do something like this...
Previously, i unset PINNED flag directly is a lazy approach, that is
because i think it
dosen't hurt to do so, because we are going to evict that inode, and after
btrfs_invalidatepages() is called, nobody should can still access those pages.
Besides, i think we don't need call btrfs_wait_ordered_extents() in
evicting inode here..
(Expecially after your patch that we remove extent map cache)....
What do you think ^_^
>
> I'm ok with your approach too.
>
> thanks
>
>
>>
>> What do you think?
>>
>> thanks
>>
>>>
>>>
>>>>
>>>> Does it makes sense?
>>>>
>>>> thanks
>>>>
>>>>>
>>>>> Thanks,
>>>>> Wang
>>>>>>
>>>>>> Thanks Wang for finding out this.
>>>>>>
>>>>>> Signed-off-by: Filipe David Borba Manana <fdmanana@xxxxxxxxx>
>>>>>> ---
>>>>>> fs/btrfs/inode.c | 5 +++--
>>>>>> 1 file changed, 3 insertions(+), 2 deletions(-)
>>>>>>
>>>>>> diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
>>>>>> index e889779..c2933fb 100644
>>>>>> --- a/fs/btrfs/inode.c
>>>>>> +++ b/fs/btrfs/inode.c
>>>>>> @@ -4509,6 +4509,9 @@ static void evict_inode_truncate_pages(struct inode *inode)
>>>>>> ASSERT(inode->i_state & I_FREEING);
>>>>>> truncate_inode_pages(&inode->i_data, 0);
>>>>>>
>>>>>> + /* do we really want it for ->i_nlink > 0 and zero btrfs_root_refs? */
>>>>>> + btrfs_wait_ordered_range(inode, 0, (u64)-1);
>>>>>> +
>>>>>> write_lock(&map_tree->lock);
>>>>>> while (!RB_EMPTY_ROOT(&map_tree->map)) {
>>>>>> struct extent_map *em;
>>>>>> @@ -4566,8 +4569,6 @@ void btrfs_evict_inode(struct inode *inode)
>>>>>> btrfs_orphan_del(NULL, inode);
>>>>>> goto no_delete;
>>>>>> }
>>>>>> - /* do we really want it for ->i_nlink > 0 and zero btrfs_root_refs? */
>>>>>> - btrfs_wait_ordered_range(inode, 0, (u64)-1);
>>>>>>
>>>>>> if (root->fs_info->log_root_recovering) {
>>>>>> BUG_ON(test_bit(BTRFS_INODE_HAS_ORPHAN_ITEM,
>>>>>> --
>>>>>> 1.7.9.5
>>>>>>
>>>>>> --
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>>>>
>>>>
>>>>
>>>> --
>>>> Filipe David Manana,
>>>>
>>>> "Reasonable men adapt themselves to the world.
>>>> Unreasonable men adapt the world to themselves.
>>>> That's why all progress depends on unreasonable men."
>>
>>
>>
>> --
>> Filipe David Manana,
>>
>> "Reasonable men adapt themselves to the world.
>> Unreasonable men adapt the world to themselves.
>> That's why all progress depends on unreasonable men."
>
>
>
> --
> Filipe David Manana,
>
> "Reasonable men adapt themselves to the world.
> Unreasonable men adapt the world to themselves.
> That's why all progress depends on unreasonable men."
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