Re: Using page-fault handler in binder

[Suren Baghdasaryan <surenb@google.com>]

On Thu, Apr 20, 2023 at 1:34 PM Carlos Llamas <cmllamas@google.com> 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