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SubjectRe: [PATCH v5 11/12] block: Add bio_clone_bioset()
On Wed, Aug 08, 2012 at 07:56:10PM -0700, Kent Overstreet wrote:
> > So, bio_clone() loses its function comment. Also, does it even make
> > sense to call bio_clone() from fs_bio_set?
>
> I'll re add the function comment if you want, just for a single line
> wrapper I don't know if it's worth the cost - comments get out of date,
> and they're more stuff to wade through.

People actually look at docbook generated docs. I don't know why but
they do. It's a utility function at block layer. Please just add the
comment.

> > Let's say it's so, then
> > what's the difference from using _kmalloc variant?
>
> bio_kmalloc() fails if nr_iovecs > 1024, bio_alloc_bioset() fails if
> nr_iovecs > 256
>
> and bio_alloc_bioset() is mempool backed, bio_kmalloc() is not.
>
> AFAICT that's it.

So, the thing is being mempool backed doesn't mean anything if
multiple layers use the pool. I *suspect* fs_bio_set is supposed to
be used by fs layer - ie. where bios originate. The reason why I
wondered about bio_clone() is that bio_clone() is almost always used
from stacking drivers and stacking driver tapping into fs reserve is
buggy. So, I'm wondering whether cloning from fs_bio_set should be
supported at all.

Thanks.

--
tejun


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