BUG? ondemand_readahead() exeeds bdi->io_pages

[Markus Stockhausen]

Hi there,

I'm investigating an problem where I see block I/O twice the maximum
allowed size in max_hw_sectors_kb. In my case the device driver simply
fails. I have to investigate that part further... Nevertheless I think there
might be some strange calculation bug in ondemand_readahead().

Assume user reads 1M of data and the device driver can only handle
this 1M. In initial_readahead we will set ra->size to 1M and async size
to 1M too.

initial_readahead:
  ra->start = offset;
  ra->size = get_init_ra_size(req_size, max_pages);
  ra->async_size = ra->size > req_size ? ra->size - req_size : ra->size;

In the readit part the check succeeds and the function will double
the amount to read ahead.

readit:
  /*
   * Will this read hit the readahead marker made by itself?
   * If so, trigger the readahead marker hit now, and merge
   * the resulted next readahead window into the current one.
   */
  if (offset == ra->start && ra->size == ra->async_size) {
    ra->async_size = get_next_ra_size(ra, max_pages);
    ra->size += ra->async_size;
  }

Btrace of dd if=/dev/fioa1 of=/dev/null bs=1M count=1 gives:

253,0   20        1     0.000000000 19784  A   R 2048 + 4096 <- (253,1) 0
253,0   20        2     0.000003330 19784  Q   R 2048 + 4096 [dd]
253,0   20        3     0.000235833 19784  C   R 2048 + 4096 [0]

Initial commit was 122a21d11... (readahead: on-demand readahead
logic) and the readit part was extended by 51daa88eb... (readahead:
remove sync/async readahead call dependency)

I'm lost where one should implement a fix. In the first part of
the calculation or the second? Or am I totally wrong?

Thanks for your help and best regards.

Markus


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