Re: [RFC PATCH] fs: block_dev: compute nr_vecs hint for improving writeback bvecs allocation

[Christoph Hellwig]

At least for iomap I think this is the wrong approach.  Between the
iomap and writeback_control we know the maximum size of the writeback
request and can just use that.  Take a look at what iomap_readpage_actor
does on the read size, something similar taking the wbc into account
can be done on the writeback side.