Re: md: Does the thread entering the wait queue violate the semantics of REQ_NOWAIT in raid5_make_request() ?

[Yu Kuai <yukuai1@huaweicloud.com>]

Hi,

在 2025/08/06 21:34, chen cheng 写道:
> static bool raid5_make_request(struct mddev *mddev, struct bio * bi)
> {
>          ...
> 
>          if ((bi->bi_opf & REQ_NOWAIT) &&
>              (conf->reshape_progress != MaxSector) &&
>              get_reshape_loc(mddev, conf, logical_sector) ==
> LOC_INSIDE_RESHAPE) {
>                  bio_wouldblock_error(bi);
>                  if (rw == WRITE)
>                          md_write_end(mddev);
>                  return true;
>          }
> 
>          if (likely(conf->reshape_progress == MaxSector)) {
>                  ...
>          } else {
>                  add_wait_queue(&conf->wait_for_reshape, &wait);
>                  on_wq = true;
>          }
> 
>          ...
> }
> 
> 
> In raid5_make_request(), if a reshape is progressing and the current
> IO request is not within the reshape range and has the REQ_NOWAIT
> flag, does the thread entering the wait queue violate the semantics of
> REQ_NOWAIT?

The mdraid really doesn't support REQ_NOWAIT, for now the flag is just
ignored and I think BLK_FEAT_NOWAIT should be cleared for raid1/10/456.

BTW, support it will require a lot of work, because IO error handling is
too complicated, we don't want to record badblocks for nowait IO
failure, however, if write disk a succeed while write disk b failed,
whatever we do will violate the semantics of nowait.

Thanks,
Kuai

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