Re: [PATCH RFC] yet more struct scsi_lun

[Mike Christie <michaelc@cs.wisc.edu>]

Mike Christie wrote:
> Jens Axboe wrote:
> 
>>
>> On the SCSI side, I would suggest just making shost->max_sectors set
>> q->max_hw_sectors and leave q->max_sectors to some generic kernel-wide
>> block layer define (of course making sure that ->max_sectors <=
> 
> 
> If the value for this block layer define was around 16,000 sectors, 
> would that be ok? The reason I ask is becuase when I get .....
> 
> 
>> ->max_hw_sectors). That's the easy part.
>>
>> The bio_add_page() stuff is a little trickier, since it wants to know if
>> this is fs or 'generic' io. For fs io, we would like to cap the building
>> of the bio to ->max_sectors, but for eg SG_IO issued io it should go as
>> high as ->max_hw_sectors. Perhaps the easiest is just to have
>> bio_fs_add_page() and bio_pc_add_page(), each just passing in the max
>> value as an integer to bio_add_page(). But it's not exactly pretty.
>>
>> The ll_rw_blk.c merging is easy, since you don't need to do anything
>> there. It should test against ->max_sectors as it already does, since
>> this (sadly) is still the primary way we build large ios.
>>
> 
> .... here, I am running into a problem. Basically, as you know the 
> largest BIO we can make is 1 MB due to BIO_MAX_PAGES, and for st and sg 
> we need to support commands around 6 MB, so we would have a request with 
>  6 BIOs. To make this monster request I wanted to use the block layer 
> functions and do something like this:
> 
> +       for (i = 0; i < nsegs; i++) {
> +               bio = bio_map_pages(q, sg[i].page, sg[i].length, 
> sg[i].offset, gfp);

oh yeah bio_map_pages, just takes an array of pages and does 
bio_*_add_page on them.

> +               if (IS_ERR(bio)) {
> +                       err = PTR_ERR(bio);
> +                       goto free_bios;
> +               }
> +               len += sg[i].length;
> +
> +               bio->bi_flags &= ~(1 << BIO_SEG_VALID);
> +               if (rq_data_dir(rq) == WRITE)
> +                       bio->bi_rw |= (1 << BIO_RW);
> +               blk_queue_bounce(q, &bio);
> +
> +               if (i == 0)
> +                       blk_rq_bio_prep(q, rq, bio);
> 
> /* hope to carve out the __make_request code that does the below 
> operations and make a fucntion that can be shared */
> 
> +               else if (!q->back_merge_fn(q, rq, bio)) {
> +                       err = -EINVAL;
> +                       bio_endio(bio, bio->bi_size, 0);
> +                       goto free_bios;
> +               } else {
> +                       rq->biotail->bi_next = bio;
> +                       rq->biotail = bio;
> +                       rq->hard_nr_sectors += bio_sectors(bio);
> +                       rq->nr_sectors = rq->hard_nr_sectors;
> 
> ........
> 
> But since q->back_merge_fn() tests against q->max_sectors, it must be a 
> high value so that we can merge in those BIOs. I mean if q->max_sectors 
> is some reasonable number like only 1024 sectors, q->back_merge_fn will 
> return a failure. Should I instead seperate ll_back_merge_fn into two 
> functions, one that checks the sectors and one that checks the segments 
> or if ll_back_merge_fn tested for max_hw_sectors we would be ok too?
>