Re: [md PATCH 1/4] md: add block tracing for bio_remapping

[NeilBrown <neilb@suse.com>]

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On Thu, Nov 17 2016, Shaohua Li wrote:

> On Mon, Nov 14, 2016 at 04:30:21PM +1100, Neil Brown wrote:
>> The block tracing infrastructure (accessed with blktrace/blkparse)
>> supports the tracing of mapping bios from one device to another.
>> This is currently used when a bio in a partition is mapped to the
>> whole device, when bios are mapped by dm, and for mapping in md/raid5.
>> Other md personalities do not include this tracing yet, so add it.
>> 
>> When a read-error is detected we redirect the request to a different device.
>> This could justifiably be seen as a new mapping for the originial bio,
>> or a secondary mapping for the bio that errors.  This patch uses
>> the second option.
>> 
>> When md is used under dm-raid, the mappings are not traced as we do
>> not have access to the block device number of the parent.
>
> Looks the the original sector (the last parameter of trace_block_bio_remap)
> isn't correct.
> - in linear/raid0, bio_split already updated bio->bi_iter.bi_sector

Oh yes, of course.  in the common case 'split == bio' so when
split->bi_iter.bi_sector  is adjusted, bio->.... is as well.
I'll fix that, and also add calls to trace_block_split() as appropriate.


> - in raid1/raid10, r1_bio->sector is updated before the bio is sent.

Here I really think my code is correct.  r1_bio->sector is always the
address in the array of the request.  It is only set once for each
r1_bio, and that is before the call to trace_block_io_remap().

Thanks,
NeilBrown


>
> Thanks,
> Shaohua

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