Re: [PATCH 0/8] Set bi_rw when alloc bio before call bio_add_page.

[majianpeng <majianpeng@gmail.com>]

On 2012-07-31 05:42 Dave Chinner <david@fromorbit.com> Wrote:
>On Mon, Jul 30, 2012 at 03:14:28PM +0800, majianpeng wrote:
>> When exec bio_alloc, the bi_rw is zero.But after calling bio_add_page,
>> it will use bi_rw.
>> Fox example, in functiion __bio_add_page,it will call merge_bvec_fn().
>> The merge_bvec_fn of raid456 will use the bi_rw to judge the merge.
>> >> if ((bvm->bi_rw & 1) == WRITE)
>> >> return biovec->bv_len; /* always allow writes to be mergeable */
>
>So if bio_add_page() requires bi_rw to be set, then shouldn't it be
>set up for every caller? I noticed there are about 50 call sites for
>bio_add_page(), and you've only touched about 10 of them. Indeed, I
>notice that the RAID0/1 code uses bio_add_page, and as that can be
>stacked on top of RAID456, it also needs to set bi_rw correctly.
>As a result, your patch set is nowhere near complete, not does it
>document that bio_add_page requires that bi_rw be set before calling
>(which is the new API requirement, AFAICT).
There are many place call bio_add_page and I send some of those. Because my abilty, so I only send 
some patchs which i understand clearly.
In __bio_add_page:
>>if (q->merge_bvec_fn) {
>>				struct bvec_merge_data bvm = {
>>					/* prev_bvec is already charged in
>>					   bi_size, discharge it in order to
>>					   simulate merging updated prev_bvec
>>					   as new bvec. */
>>					.bi_bdev = bio->bi_bdev,
>>					.bi_sector = bio->bi_sector,
>>					.bi_size = bio->bi_size - prev_bv_len,
>>					.bi_rw = bio->bi_rw,
>>				};
it used bio->bi_rw.
Before raid5_mergeable_bvec appearing, in kernel 'merge_bvec_fn' did not use bio->bi_rw.
But i think we shold not suppose bi_rw not meanless.
And I think we not need an new API to do.
Most used bio_alloc and bio_add_page, like this:
>>	bio = bio_alloc(gfp_mask, 1);
>>		if (!bio) 
>>			ret = -ENOMEM;
			
>>		bio->bi_sector = sector;
>>		bio->bi_end_io = bio_batch_end_io;
>>		bio->bi_bdev = bdev;
>>		bio->bi_private = &bb;
We only add bio->bi_rw = value;
But we shold modify Document for this.


>
>So, my question is whether the RAID456 code is doing something
>valid.  That write optimisation is clearly not enabled for a
>significant amount of code and filesystems, so the first thing to do
>is quantify the benefit of the optimisation. I can't evalute the
>merit of this change without data telling me it is worthwhile, and
>it's a lot of code to churn for no benefit....
>
Sorry, we do not think the 'merge_bvec_fn' did not use bi_rw.
>Cheers,
>
>Dave.
>-- 
>Dave Chinner
>david@fromorbit.com
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