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:

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On Wed, 2023-12-20 at 17:27 -0800, Song Liu wrote:

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you have verified the sender or the content.
 A recent bug report [1] shows md is handling a flush from

bcachefs

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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

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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$

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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,

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then we should retrieve all REQ_PREFLUSH to confirm. At least for

now,

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it seems that REQ_OP_FLUSH without REQ_PREFLUSH in

`blk_flush_policy`

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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.

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.