Re: [PATCH v2] btrfs: transaction: Commit transaction more frequently for BPF

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

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On 2019/8/16 下午5:33, Filipe Manana wrote:
> On Thu, Aug 15, 2019 at 9:36 AM Qu Wenruo <wqu@suse.com> wrote:
>>
>> Btrfs has btrfs_end_transaction_throttle() which could try to commit
>> transaction when needed.
>>
>> However under most cases btrfs_end_transaction_throttle() won't really
>> commit transaction, due to the hard timing requirement.
>>
>> Now introduce a new error injection point, btrfs_need_trans_pressure(),
>> to allow btrfs_should_end_transaction() to return 1 and
>> btrfs_end_transaction_throttle() to fallback to
>> btrfs_commit_transaction().
>>
>> With such more aggressive transaction commit, we can dig deeper into
>> cases like snapshot drop.
>> Now each reference drop of btrfs_drop_snapshot() will lead to a
>> transaction commit, allowing dm-logwrites to catch more details, other
>> than one big transaction dropping everything.
>>
>> Signed-off-by: Qu Wenruo <wqu@suse.com>
>> ---
>> Changelog:
>> v2:
>> - Add comment to explain why this function is needed
>> ---
>>  fs/btrfs/transaction.c | 18 ++++++++++++++++++
>>  1 file changed, 18 insertions(+)
>>
>> diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
>> index 3f6811cdf803..8c5471b01d03 100644
>> --- a/fs/btrfs/transaction.c
>> +++ b/fs/btrfs/transaction.c
>> @@ -10,6 +10,7 @@
>>  #include <linux/pagemap.h>
>>  #include <linux/blkdev.h>
>>  #include <linux/uuid.h>
>> +#include <linux/error-injection.h>
>>  #include "ctree.h"
>>  #include "disk-io.h"
>>  #include "transaction.h"
>> @@ -749,10 +750,25 @@ void btrfs_throttle(struct btrfs_fs_info *fs_info)
>>         wait_current_trans(fs_info);
>>  }
>>
>> +/*
>> + * This function is to allow BPF to override the return value so that we can
>> + * make btrfs to commit transaction more aggressively.
>> + *
>> + * It's a debug only feature, mainly used with dm-log-writes to catch more details
>> + * of transient operations like balance and subvolume drop.
> 
> Transient? I think you mean long running operations that can span
> multiple transactions.
> 
>> + */
>> +static noinline bool btrfs_need_trans_pressure(struct btrfs_trans_handle *trans)
>> +{
>> +       return false;
>> +}
>> +ALLOW_ERROR_INJECTION(btrfs_need_trans_pressure, TRUE);
> 
> So, I'm not sure if it's really a good idea to have such specific
> things like this.
> Has this proven useful already? I.e., have you already found any bug using this?
> 
> I often add such similar things myself for testing and debugging, but
> because they are so specific, or ugly or verbose, I keep them to
> myself.
> 
> Allowing the return value of should_end_transaction() to be
> overridden, using the same approach, would be more generic for
> example.

Forgot to mention there is another function checking this:
__btrfs_end_transaction.

If btrfs_need_trans_pressure() returns true, __btrfs_end_transaction()
will try to commit transaction for btrfs_end_transaction_throttle().

Thanks,
Qu

> 
> Thanks.
> 
>> +
>>  static int should_end_transaction(struct btrfs_trans_handle *trans)
>>  {
>>         struct btrfs_fs_info *fs_info = trans->fs_info;
>>
>> +       if (btrfs_need_trans_pressure(trans))
>> +               return 1;
>>         if (btrfs_check_space_for_delayed_refs(fs_info))
>>                 return 1;
>>
>> @@ -813,6 +829,8 @@ static int __btrfs_end_transaction(struct btrfs_trans_handle *trans,
>>
>>         btrfs_trans_release_chunk_metadata(trans);
>>
>> +       if (throttle && btrfs_need_trans_pressure(trans))
>> +               return btrfs_commit_transaction(trans);
>>         if (lock && READ_ONCE(cur_trans->state) == TRANS_STATE_BLOCKED) {
>>                 if (throttle)
>>                         return btrfs_commit_transaction(trans);
>> --
>> 2.22.0
>>
> 
> 


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