On Wed, Feb 20, 2019 at 07:19:30AM -0800, Christoph Hellwig wrote: > On Tue, Feb 19, 2019 at 09:30:33PM -0800, ira.weiny@xxxxxxxxx wrote: > > From: Ira Weiny <ira.weiny@xxxxxxxxx> > > > > Resending these as I had only 1 minor comment which I believe we have covered > > in this series. I was anticipating these going through the mm tree as they > > depend on a cleanup patch there and the IB changes are very minor. But they > > could just as well go through the IB tree. > > > > NOTE: This series depends on my clean up patch to remove the write parameter > > from gup_fast_permitted()[1] > > > > HFI1, qib, and mthca, use get_user_pages_fast() due to it performance > > advantages. These pages can be held for a significant time. But > > get_user_pages_fast() does not protect against mapping of FS DAX pages. > > This I don't get - if you do lock down long term mappings performance > of the actual get_user_pages call shouldn't matter to start with. > > What do I miss? A couple of points. First "longterm" is a relative thing and at this point is probably a misnomer. This is really flagging a pin which is going to be given to hardware and can't move. I've thought of a couple of alternative names but I think we have to settle on if we are going to use FL_LAYOUT or something else to solve the "longterm" problem. Then I think we can change the flag to a better name. Second, It depends on how often you are registering memory. I have spoken with some RDMA users who consider MR in the performance path... For the overall application performance. I don't have the numbers as the tests for HFI1 were done a long time ago. But there was a significant advantage. Some of which is probably due to the fact that you don't have to hold mmap_sem. Finally, architecturally I think it would be good for everyone to use *_fast. There are patches submitted to the RDMA list which would allow the use of *_fast (they reworking the use of mmap_sem) and as soon as they are accepted I'll submit a patch to convert the RDMA core as well. Also to this point others are looking to use *_fast.[2] As an asside, Jasons pointed out in my previous submission that *_fast and *_unlocked look very much the same. I agree and I think further cleanup will be coming. But I'm focused on getting the final solution for DAX at the moment. Ira
