Re: [PATCH 3/3] rust: add abstraction for `struct page`

[Carlos Llamas]

On Mon, Jan 29, 2024 at 05:59:53PM +0000, Matthew Wilcox wrote:
> On Wed, Jan 24, 2024 at 11:20:23AM +0000, Alice Ryhl wrote:
> > Adds a new struct called `Page` that wraps a pointer to `struct page`.
> > This struct is assumed to hold ownership over the page, so that Rust
> > code can allocate and manage pages directly.
> 
> OK ...
> 
> > This patch only adds support for pages of order zero, as that is all
> > Rust Binder needs. However, it is written to make it easy to add support
> > for higher-order pages in the future. To do that, you would add a const
> > generic parameter to `Page` that specifies the order. Most of the
> > methods do not need to be adjusted, as the logic for dealing with
> > mapping multiple pages at once can be isolated to just the
> > `with_pointer_into_page` method. Finally, the struct can be renamed to
> > `Pages<ORDER>`, and the type alias `Page = Pages<0>` can be introduced.
> 
> This description concerns me because it reads like you're not keeping
> up with the current thinking in MM about what pages are and how we're
> improving the type hierarchy.  As in, we're creating one instead of
> allowing the current mish-mash of absolutely everything to continue.
> 
> Are you the right person to ask about the operations that Binder does
> with a page so we can figure out where it fits in the type hierarchy?
 
I would guess you are suggesting a transition to folios here? I don't
think there is anything in binder that would impede such a change. The
core idea behind binder IPC is to skip kernel buffering and perform
instead a "copy-once" of messages across users memory. In theory this
seems efficient but I haven't seen any data proving so. So take that
with a grain of salt.

The size of these binder messages is not limited per se and can trigger
the allocation of multiple pages. However, in reality the vast majority
of transactions are under 1K payload. FWICT, it seems reasonable to
switch over to folios.

The only concern I have is that we've implemented a binder LRU-shrinker
mechanism. We add the unused pages to our freelist and give them back to
the system on demand. However, if a new transaction requests the unused
page before it gets reclaimed it is simply removed from this freelist.
This is convenient as we avoid taking the mmap sem during this process.
I don't know how this mechanism would look with folios though?

--
Carlos Llamas