On Mon, Mar 20, 2023 at 11:30:00AM -0700, Andrei Vagin wrote: > On Thu, Mar 9, 2023 at 11:58 AM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote: > > > > On Thu, 9 Mar 2023 18:57:11 +0500 Muhammad Usama Anjum <usama.anjum@xxxxxxxxxxxxx> wrote: > > > > > The information related to pages if the page is file mapped, present and > > > swapped is required for the CRIU project [5][6]. The addition of the > > > required mask, any mask, excluded mask and return masks are also required > > > for the CRIU project [5]. > > > > It's a ton of new code and what I'm not seeing in here (might have > > missed it?) is a clear statement of the value of this feature to our > > users. > > > > I see hints that CRIU would like it, but no description of how valuable > > this is to CRIU's users. > > Hi Andrew, > > The current interface works for CRIU, and I can't say we have anything > critical with it right now. > > On the other hand, the new interface has a number of significant improvements: > > * it is more granular and allows us to track changed pages more > effectively. The current interface can clear dirty bits for the entire > process only. In addition, reading info about pages is a separate > operation. It means we must freeze the process to read information > about all its pages, reset dirty bits, only then we can start dumping > pages. The information about pages becomes more and more outdated, > while we are processing pages. The new interface solves both these > downsides. First, it allows us to read pte bits and clear the > soft-dirty bit atomically. It means that CRIU will not need to freeze > processes to pre-dump their memory. Second, it clears soft-dirty bits > for a specified region of memory. It means CRIU will have actual info > about pages to the moment of dumping them. > > * The new interface has to be much faster because basic page filtering > is happening in the kernel. With the old interface, we have to read > pagemap for each page. There is still a caveat in using userfaultfd for tracking dirty pages in CRIU because we still don't support C/R of processes that use uffd. > Thanks, > Andrei > > > > > So please spend some time preparing this info. > > > > Also, are any other applications of this feature anticipated? If so, > > what are they? > > > > IOW, please sell this stuff to us! -- Sincerely yours, Mike.
