Messages in this thread | | | Date | Wed, 8 Aug 2012 20:06:38 -0700 | From | Kent Overstreet <> | Subject | Re: [PATCH v5 12/12] block: Only clone bio vecs that are in use |
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On Wed, Aug 08, 2012 at 04:30:07PM -0700, Tejun Heo wrote: > Hello, > > On Mon, Aug 06, 2012 at 03:08:41PM -0700, Kent Overstreet wrote: > > @@ -459,10 +460,10 @@ void __bio_clone(struct bio *bio, struct bio *bio_src) > > bio->bi_sector = bio_src->bi_sector; > > bio->bi_bdev = bio_src->bi_bdev; > > bio->bi_flags |= 1 << BIO_CLONED; > > + bio->bi_flags &= ~(1 << BIO_SEG_VALID); > > This isn't obvious at all. Why no explanation anywhere? Also it > would be nice to update comments of the updated functions so that it's > clear that only partial cloning happens.
Because it's not obvious to me, either - I had to grep around through a fair amount of code to figure out the semantics of BIO_SEG_VALID and I doubt I have it 100%. I'm also pretty sure it's not used consistently in the existing code...
If it means what I think it means, it should be obvious - nr_segs isn't valid because the number of pages in the iovec changed (though we didn't bother to copy it over anyways. So it's not necessary if nr_segs = 0 means nr_segs isn't valid, but bleh).
Anyways. yeah. BIO_SEG_VALID should be documented, and if it was I think this code would be fine.
I will update the comment for the partial cloning thing:
/** * __bio_clone - clone a bio * @bio: destination bio * @bio_src: bio to clone * * Clone a &bio. Caller will own the returned bio, but not * the actual data it points to. Reference count of returned * bio will be one. * * We don't clone the entire bvec, just the part from bi_idx to b_vcnt * (i.e. what the bio currently points to, so the new bio is still * equivalent to the old bio). */ void __bio_clone(struct bio *bio, struct bio *bio_src) { memcpy(bio->bi_io_vec, bio_iovec(bio_src), bio_segments(bio_src) * sizeof(struct bio_vec));
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