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
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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