On 01/12/2020 12:59, Christoph Hellwig wrote: > On Tue, Dec 01, 2020 at 12:52:51PM +0000, Matthew Wilcox wrote: >> But the only reason we want to know 'nr_vecs' is so we can allocate a >> BIO which has that many vecs, right? But we then don't actually use the >> vecs in the bio because we use the ones already present in the iter. >> That was why I had it return 1, not nr_vecs. >> >> Did I miss something? > > Right now __bio_iov_bvec_add_pages does not reuse the bvecs in the > iter. That being said while we are optmizing this path we might a well > look into reusing them.. I was thinking about memcpy bvec instead of iterating as a first step, and then try to reuse passed in bvec. A thing that doesn't play nice with that is setting BIO_WORKINGSET in __bio_add_page(), which requires to iterate all pages anyway. I have no clue what it is, so rather to ask if we can optimise it out somehow? Apart from pre-computing for specific cases... E.g. can pages of a single bvec segment be both in and out of a working set? (i.e. PageWorkingset(page)). -- Pavel Begunkov
