Re: [PATCH v4 05/18] btrfs: delayed-ref: Add support for atomic increasing extent ref

[Qu Wenruo <quwenruo@cn.fujitsu.com>]

Hi Filipe,

Thanks for your review.

Filipe Manana wrote on 2016/01/14 09:56 +0000:
> On Thu, Jan 14, 2016 at 5:57 AM, Qu Wenruo <quwenruo@cn.fujitsu.com> wrote:
>> Slightly modify btrfs_add_delayed_data_ref() to allow it accept
>> GFP_ATOMIC, and allow it to do be called inside a spinlock.
>
> Hi Qu,
>
> I really would prefer to avoid gfp_atomic allocations.
> Instead of using them, how about changing the approach so that callers
> allocate the ref structure (without using gfp_atomic), then take the
> spinlock, and then pass the structure to the inc/dec ref functions?

Yes, we also considered that method too.

But since it's only used for dedup hit case which has no existing 
delayed ref head, it's on a uncommon routine.

On the other hand, if using the method you mentioned, alloc memory first 
then pass it to add_delayed_data_ref(), we will always need to alloc 
memory for minority cases.

It would be very nice if you can provide some extra disadvantage on 
using GFP_ATOMIC, maybe there is something important I missed.

Thanks
Qu

>
> thanks
>
>>
>> This is used by later dedup patches.
>>
>> Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
>> ---
>>   fs/btrfs/ctree.h       |  4 ++++
>>   fs/btrfs/delayed-ref.c | 25 +++++++++++++++++--------
>>   fs/btrfs/delayed-ref.h |  2 +-
>>   fs/btrfs/extent-tree.c | 24 +++++++++++++++++++++---
>>   4 files changed, 43 insertions(+), 12 deletions(-)
>>
>> diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
>> index 2132fa5..671be87 100644
>> --- a/fs/btrfs/ctree.h
>> +++ b/fs/btrfs/ctree.h
>> @@ -3539,6 +3539,10 @@ int btrfs_inc_extent_ref(struct btrfs_trans_handle *trans,
>>                           struct btrfs_root *root,
>>                           u64 bytenr, u64 num_bytes, u64 parent,
>>                           u64 root_objectid, u64 owner, u64 offset);
>> +int btrfs_inc_extent_ref_atomic(struct btrfs_trans_handle *trans,
>> +                               struct btrfs_root *root, u64 bytenr,
>> +                               u64 num_bytes, u64 parent,
>> +                               u64 root_objectid, u64 owner, u64 offset);
>>
>>   int btrfs_start_dirty_block_groups(struct btrfs_trans_handle *trans,
>>                                     struct btrfs_root *root);
>> diff --git a/fs/btrfs/delayed-ref.c b/fs/btrfs/delayed-ref.c
>> index 914ac13..e869442 100644
>> --- a/fs/btrfs/delayed-ref.c
>> +++ b/fs/btrfs/delayed-ref.c
>> @@ -812,26 +812,31 @@ int btrfs_add_delayed_data_ref(struct btrfs_fs_info *fs_info,
>>                                 u64 bytenr, u64 num_bytes,
>>                                 u64 parent, u64 ref_root,
>>                                 u64 owner, u64 offset, u64 reserved, int action,
>> -                              struct btrfs_delayed_extent_op *extent_op)
>> +                              int atomic)
>>   {
>>          struct btrfs_delayed_data_ref *ref;
>>          struct btrfs_delayed_ref_head *head_ref;
>>          struct btrfs_delayed_ref_root *delayed_refs;
>>          struct btrfs_qgroup_extent_record *record = NULL;
>> +       gfp_t gfp_flags;
>> +
>> +       if (atomic)
>> +               gfp_flags = GFP_ATOMIC;
>> +       else
>> +               gfp_flags = GFP_NOFS;
>>
>> -       BUG_ON(extent_op && !extent_op->is_data);
>> -       ref = kmem_cache_alloc(btrfs_delayed_data_ref_cachep, GFP_NOFS);
>> +       ref = kmem_cache_alloc(btrfs_delayed_data_ref_cachep, gfp_flags);
>>          if (!ref)
>>                  return -ENOMEM;
>>
>> -       head_ref = kmem_cache_alloc(btrfs_delayed_ref_head_cachep, GFP_NOFS);
>> +       head_ref = kmem_cache_alloc(btrfs_delayed_ref_head_cachep, gfp_flags);
>>          if (!head_ref) {
>>                  kmem_cache_free(btrfs_delayed_data_ref_cachep, ref);
>>                  return -ENOMEM;
>>          }
>>
>>          if (fs_info->quota_enabled && is_fstree(ref_root)) {
>> -               record = kmalloc(sizeof(*record), GFP_NOFS);
>> +               record = kmalloc(sizeof(*record), gfp_flags);
>>                  if (!record) {
>>                          kmem_cache_free(btrfs_delayed_data_ref_cachep, ref);
>>                          kmem_cache_free(btrfs_delayed_ref_head_cachep,
>> @@ -840,10 +845,13 @@ int btrfs_add_delayed_data_ref(struct btrfs_fs_info *fs_info,
>>                  }
>>          }
>>
>> -       head_ref->extent_op = extent_op;
>> +       head_ref->extent_op = NULL;
>>
>>          delayed_refs = &trans->transaction->delayed_refs;
>> -       spin_lock(&delayed_refs->lock);
>> +
>> +       /* For atomic case, caller should already hold the delayed_refs lock */
>> +       if (!atomic)
>> +               spin_lock(&delayed_refs->lock);
>>
>>          /*
>>           * insert both the head node and the new ref without dropping
>> @@ -856,7 +864,8 @@ int btrfs_add_delayed_data_ref(struct btrfs_fs_info *fs_info,
>>          add_delayed_data_ref(fs_info, trans, head_ref, &ref->node, bytenr,
>>                                     num_bytes, parent, ref_root, owner, offset,
>>                                     action);
>> -       spin_unlock(&delayed_refs->lock);
>> +       if (!atomic)
>> +               spin_unlock(&delayed_refs->lock);
>>
>>          return 0;
>>   }
>> diff --git a/fs/btrfs/delayed-ref.h b/fs/btrfs/delayed-ref.h
>> index c24b653..e34f96a 100644
>> --- a/fs/btrfs/delayed-ref.h
>> +++ b/fs/btrfs/delayed-ref.h
>> @@ -249,7 +249,7 @@ int btrfs_add_delayed_data_ref(struct btrfs_fs_info *fs_info,
>>                                 u64 bytenr, u64 num_bytes,
>>                                 u64 parent, u64 ref_root,
>>                                 u64 owner, u64 offset, u64 reserved, int action,
>> -                              struct btrfs_delayed_extent_op *extent_op);
>> +                              int atomic);
>>   int btrfs_add_delayed_qgroup_reserve(struct btrfs_fs_info *fs_info,
>>                                       struct btrfs_trans_handle *trans,
>>                                       u64 ref_root, u64 bytenr, u64 num_bytes);
>> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
>> index 60cc139..4a01ca9 100644
>> --- a/fs/btrfs/extent-tree.c
>> +++ b/fs/btrfs/extent-tree.c
>> @@ -2105,11 +2105,29 @@ int btrfs_inc_extent_ref(struct btrfs_trans_handle *trans,
>>                  ret = btrfs_add_delayed_data_ref(fs_info, trans, bytenr,
>>                                          num_bytes, parent, root_objectid,
>>                                          owner, offset, 0,
>> -                                       BTRFS_ADD_DELAYED_REF, NULL);
>> +                                       BTRFS_ADD_DELAYED_REF, 0);
>>          }
>>          return ret;
>>   }
>>
>> +int btrfs_inc_extent_ref_atomic(struct btrfs_trans_handle *trans,
>> +                               struct btrfs_root *root, u64 bytenr,
>> +                               u64 num_bytes, u64 parent,
>> +                               u64 root_objectid, u64 owner, u64 offset)
>> +{
>> +       struct btrfs_fs_info *fs_info = root->fs_info;
>> +
>> +       BUG_ON(owner < BTRFS_FIRST_FREE_OBJECTID &&
>> +              root_objectid == BTRFS_TREE_LOG_OBJECTID);
>> +
>> +       /* Only used by dedup, so only data is possible */
>> +       if (WARN_ON(owner < BTRFS_FIRST_FREE_OBJECTID))
>> +               return -EINVAL;
>> +       return btrfs_add_delayed_data_ref(fs_info, trans, bytenr,
>> +                       num_bytes, parent, root_objectid,
>> +                       owner, offset, 0, BTRFS_ADD_DELAYED_REF, 1);
>> +}
>> +
>>   static int __btrfs_inc_extent_ref(struct btrfs_trans_handle *trans,
>>                                    struct btrfs_root *root,
>>                                    struct btrfs_delayed_ref_node *node,
>> @@ -6893,7 +6911,7 @@ int btrfs_free_extent(struct btrfs_trans_handle *trans, struct btrfs_root *root,
>>                                                  num_bytes,
>>                                                  parent, root_objectid, owner,
>>                                                  offset, 0,
>> -                                               BTRFS_DROP_DELAYED_REF, NULL);
>> +                                               BTRFS_DROP_DELAYED_REF, 0);
>>          }
>>          return ret;
>>   }
>> @@ -7845,7 +7863,7 @@ int btrfs_alloc_reserved_file_extent(struct btrfs_trans_handle *trans,
>>                                           ins->offset, 0,
>>                                           root_objectid, owner, offset,
>>                                           ram_bytes, BTRFS_ADD_DELAYED_EXTENT,
>> -                                        NULL);
>> +                                        0);
>>          return ret;
>>   }
>>
>> --
>> 2.7.0
>>
>>
>>
>> --
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>
>
>