Re: [PATCH 0/2] block, md: Better handle REQ_OP_FLUSH

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On Wed, 2023-12-20 at 17:27 -0800, Song Liu wrote:
>  	 
> External email : Please do not click links or open attachments until
> you have verified the sender or the content.
>  A recent bug report [1] shows md is handling a flush from bcachefs
> as read:
> 
> bch2_journal_write=>
>   submit_bio=>
>     ...
>     md_handle_request =>
>       raid5_make_request =>
>         chunk_aligned_read =>
>           raid5_read_one_chunk =>
> 	    ...
> 
> It appears md code only checks REQ_PREFLUSH for flush requests, which
> doesn't cover all cases. OTOH, op_is_flush() doesn't check
> REQ_OP_FLUSH
> either.
> 
> Fix this by:
> 1) Check REQ_PREFLUSH in op_is_flush();
> 2) Use op_is_flush() in md code.
> 
> Thanks,
> Song
> 
> [1] 
> https://urldefense.com/v3/__https://bugzilla.kernel.org/show_bug.cgi?id=218184__;!!CTRNKA9wMg0ARbw!gQbjtS_f5d3Du2prpIT8zUM4mkZf7qDleyaAuEfG8j5tMrDvw7cfJUB04VWl0uVAL4BJ4YWbVopp$
> 

REQ_OP_FLUSH is only used by the block layer's flush code, and the
filesystem should use REQ_PREFLUSH with an empty write bio.

If we want upper layer to be able to directly send REQ_OP_FLUSH bio,
then we should retrieve all REQ_PREFLUSH to confirm. At least for now,
it seems that REQ_OP_FLUSH without REQ_PREFLUSH in `blk_flush_policy`
will directly return 0 and no flush operation will be sent to the
driver.

>  
> 
> Song Liu (2):
>   block: Check REQ_OP_FLUSH in op_is_flush()
>   md: Use op_is_flush() to check flush bio
> 
>  drivers/md/raid0.c        | 2 +-
>  drivers/md/raid1.c        | 2 +-
>  drivers/md/raid10.c       | 2 +-
>  drivers/md/raid5.c        | 2 +-
>  include/linux/blk_types.h | 3 ++-
>  5 files changed, 6 insertions(+), 5 deletions(-)
> 
> --
> 2.34.1




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