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?
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