On 9/9/21 9:27 PM, Al Viro wrote: > On Thu, Sep 09, 2021 at 09:22:30PM -0600, Jens Axboe wrote: >> On 9/9/21 9:11 PM, Al Viro wrote: >>> On Thu, Sep 09, 2021 at 09:05:13PM -0600, Jens Axboe wrote: >>>> On 9/9/21 8:57 PM, Al Viro wrote: >>>>> On Thu, Sep 09, 2021 at 03:19:56PM -0600, Jens Axboe wrote: >>>>> >>>>>> Not sure how we'd do that, outside of stupid tricks like copy the >>>>>> iov_iter before we pass it down. But that's obviously not going to be >>>>>> very efficient. Hence we're left with having some way to reset/reexpand, >>>>>> even in the presence of someone having done truncate on it. >>>>> >>>>> "Obviously" why, exactly? It's not that large a structure; it's not >>>>> the optimal variant, but I'd like to see profiling data before assuming >>>>> that it'll cause noticable slowdowns. >>>> >>>> It's 48 bytes, and we have to do it upfront. That means we'd be doing it >>>> for _all_ requests, not just when we need to retry. As an example, current >>>> benchmarks are at ~4M read requests per core. That'd add ~200MB/sec of >>>> memory traffic just doing this copy. >>> >>> Umm... How much of that will be handled by cache? >> >> Depends? And what if the iovec itself has been modified in the middle? >> We'd need to copy that whole thing too. It's just not workable as a >> solution. > > Huh? Why the hell would we need to copy iovecs themselves? They are > never modified by ->read_iter()/->write_iter(). > > That's the whole fucking point of iov_iter - the iovec itself is made > constant, with all movable parts taken to iov_iter. > > Again, we should never, ever modify the iovec (or bvec, etc.) array in > ->read_iter()/->write_iter()/->sendmsg()/etc. instances. If you see > such behaviour anywhere, report it immediately. Any such is a blatant > bug. Yes that was wrong, the iovec is obviously const. But that really doesn't change the original point, which was that copying the iov_iter itself unconditionally would be miserable. -- Jens Axboe
