On Tue, Mar 21, 2023 at 02:41:53PM +0200, Mike Rapoport wrote: > 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 The information about pages becomes more and more outdated, 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. This reminded me whether the interface can also expose soft-dirty as a ranged soft-dirty collector too to replace existing pagemap read()s? Just in case userfault cannot be used. The code addition should be trivial IIUC. Then maybe PAGE_IS_WRITTEN will be a name too generic, it can be two bits PAGE_IS_UFFD_WP and PAGE_IS_SOFT_DIRTY, having PAGE_IS_UFFD_WP the inverted meaning of current PAGE_IS_WRITTEN. -- Peter Xu
