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

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On Thu, Dec 21, 2023 at 07:56:45AM +0000, Ed Tsai (蔡宗軒) wrote:
> On Thu, 2023-12-21 at 00:30 -0500, Kent Overstreet wrote:
> >  On Thu, Dec 21, 2023 at 03:36:40AM +0000, Ed Tsai (蔡宗軒) wrote:
> > > On Wed, 2023-12-20 at 17:27 -0800, Song Liu wrote:
> > > > 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.
> > 
> > If that's the case, then it should be documented and there should be
> > a
> > WARN_ON() in generic_make_request().
> 
> Please refer to the writeback_cache_control.rst. Use an empty write bio
> with the REQ_PREFLUSH flag for an explicit flush, or as commonly
> practiced by most filesystems, use blkdev_issue_flush for a pure flush.

That's not a substitute for a proper comment in the code.

> 
> > 
> > Also, glancing at blk_types.h, we have the req_op and req_flag_bits
> > both
> > using (__force blk_opf_t), but using the same bit range - what the
> > hell?
> > That's seriously broken...
> 
> No, read the comment before req_op. We do not need to use the entire 32
> bits to represent OP; only 8 bits for OP, while the remaning 24 bits is
> used for FLAG.

No, this is just broken; it's using the same bitwise enum for two
different enums.

bitwise exists for a reason - C enums are not natively type safe, and
mixing up enums/bitflags and using them in the wrong context is a
serious source of bugs. If it would be incorrect to or the two different
flags together, you can't use the same bitwise type.




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