Re: [PATCH RFC] block: fix bio merge checks when virt_boundary is set

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Keith Busch <keith.busch@intel.com> writes:

> On Tue, Mar 15, 2016 at 04:17:56PM +0100, Vitaly Kuznetsov wrote:
>> The reason of the slowdown is the fact that bios don't get merged and we
>> end up sending many short requests to the host. My investigation led me to
>> the following code (__bvec_gap_to_prev()):
>> 
>>     return offset ||
>>            ((bprv->bv_offset + bprv->bv_len) & queue_virt_boundary(q));
>> 
>> Here is an example: we have two bio_vec with the following content:
>>     bprv.bv_offset = 512
>>     bprv.bv_len = 512
>> 
>>     bnxt.bv_offset = 1024
>>     bnxt.bv_len = 512
>> 
>>     bprv.bv_page == bnxt.bv_page
>>     virt_boundary is set to PAGE_SIZE-1
>> 
>> The above mentioned code will report that a gap will appear if we merge
>> these two (as offset = 1024) but this doesn't look sane. On top of that,
>> we have the following optimization in bio_add_pc_page():
>> 
>>     if (page == prev->bv_page &&
>>         offset == prev->bv_offset + prev->bv_len) {
>>             prev->bv_len += len;
>>             bio->bi_iter.bi_size += len;
>>             goto done;
>>         }
>
> This part sounds odd. Why is a filesystem using bio_add_pc_page? Shouldn't
> these go through "bio_add_page" instead? That already has an optimization
> to combine bio's within the same page.

Not sure I know enough to comment here and it is most probably unrelated
to the issue I'm seeing (bio_add_pc_page() doesn't pop up when I do
'mkfs.ntfs') but in this particular place I see same page check before
we do bvec_gap_to_prev() but there is no such check in other places and
bios in the same page are always being split:

return offset || ((bprv->bv_offset + bprv->bv_len) & queue_virt_boundary(q));

will always return 'true' because offset is the offset of the second
bio. That's what I'm trying to address.

-- 
  Vitaly