On Thu, Apr 20, 2023 at 1:34 PM Carlos Llamas <cmllamas@xxxxxxxxxx> wrote: > > Hi, > > I'm trying to remove the current page handling in binder and switch to > using ->fault() and other mm/ infrastructure. See, we currently allocate > and insert pages manually into vmas, so dropping this boilerplate code > seems like a great idea to me. Before starting on this though, I could > use some pointers on some of the implementation details. > > In binder the messages are not buffered. What happens is that a server > task mmaps a region and then allows clients to write their user data > directly into this _remote_ vma. This is currently achieved by caching > pointers to every page allocated in the server's region. > > Instead of caching page pointers, I believe binder could make use of > get_user_pages_remote() and trigger a page fault as needed. Basically, > implement something similar to access_remote_vm() but that instead does > a copy_from_user(). However, I don't see many in-tree users of these > routines, so I wonder if I'm overlooking something in this approach? Sounds doable but there might be GUP details I'm missing... > > One more reason for the page caching in binder is being able to free > pages from the shrinker's callback. Unused pages are added to an LRU > list and we manually zap/free them when they are reclaimed. It is not > evident to me how can binder mark a range within the vma region as > reclaimable? I think you would have to release the individual pages using put_page(). Also note that get_user_pages_remote() is being deprecated, see: https://elixir.bootlin.com/linux/latest/source/mm/gup.c#L2171 > > Any pointers are appreciated! > > -- > Carlos Llamas
