On Thu, 2018-07-19 at 17:11 +0200, Christoph Hellwig wrote: > On Thu, Jul 19, 2018 at 05:06:42PM +0200, Jan Kara wrote: > > Yeah. Actually previous version of the fix (not posted publicly) > > submitted > > partial bio and then reused the bio to submit more. This is also > > the way > > __blkdev_direct_IO operates. Martin optimized this to fill the > > bio > > completely (as we know we have enough bvecs) before submitting > > which has > > chances to perform better. I'm fine with either approach, just we > > have to > > decide which way to go. > > I think this first version is going to be less fragile, so I we > should > aim for that. Wait a second. We changed the approach for a reason. By submitting partial bios synchronously and sequentially, we loose a lot of performance, so much that this "fast path" quickly falls behind the regular __blkkdev_direct_IO() path as the number of input IO segments increases. The patch I submitted avoids that. The whole point of this fast path is to submit a single bio, and return as quickly as possible. It's not an error condition if bio_iov_iter_get_pages() returns less data than possible. The function just takes one iteration step at a time, and thus iterating over it until the desired number of pages is obtained, and collecting the resulting pages in a single bio, isn't "fragile", it's just the natural thing to do. Martin -- Dr. Martin Wilck <mwilck@xxxxxxxx>, Tel. +49 (0)911 74053 2107 SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton HRB 21284 (AG Nürnberg)
