Hi,

On Mon, Jul 30, 2012 at 6:14 PM, Dave Chinner [off-list ref] wrote:

On Tue, Jul 31, 2012 at 08:55:59AM +0800, majianpeng wrote:

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On 2012-07-31 05:42 Dave Chinner [off-list ref] Wrote:

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On Mon, Jul 30, 2012 at 03:14:28PM +0800, majianpeng wrote:

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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.

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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

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some patchs which i understand clearly.

Sure, but my point is that there is no point changing only a few and
ignoring the great majority of callers. Either fix them all, fix it
some other way (e.g. API change), or remove the code from the RAID5
function that requires it.

A while back, we tried to address this by changing the alloc functions to
take rw argument and set it (as per Jens suggestion). I guess the patch did
not make it in. Please check:

https://lkml.org/lkml/2011/7/11/275

And the follow ups. If needed, I can dust up that patch and resend it.

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In __bio_add_page:

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if (q->merge_bvec_fn) {
                           struct bvec_merge_data bvm = {
                                   /* prev_bvec is already charged in
                                      bi_size, discharge it in order

to

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                                      simulate merging updated

prev_bvec

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                                      as new bvec. */
                                   .bi_bdev = bio->bi_bdev,
                                   .bi_sector = bio->bi_sector,
                                   .bi_size = bio->bi_size -

prev_bv_len,

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                                   .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.

<snip>

It's entirely possible that when bi_rw was added to struct
bvec_merge_data, the person who added it was mistaken that bi_rw was
set at this point in time when in fact it never has been. Hence it's
presence and reliance on it would be a bug.

That's what I'm asking - is this actually beneificial, or should it
simply be removed from struct bvec_merge_data? Data is needed to
answer that question....

There are cases where we found it really beneficial to know the rw field to
decide if the can be really merged or not.

Regards,
Muthu

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com
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