linux-kernel - Re: [PATCH v2 23/28] binder: document the final page calculation

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Date:   Mon, 4 Dec 2023 15:43:54 +0100
From:   Alice Ryhl <aliceryhl@...gle.com>
To:     Carlos Llamas <cmllamas@...gle.com>
Cc:     arve@...roid.com, brauner@...nel.org, gregkh@...uxfoundation.org,
        joel@...lfernandes.org, kernel-team@...roid.com,
        linux-kernel@...r.kernel.org, maco@...roid.com, surenb@...gle.com,
        tkjos@...roid.com
Subject: Re: [PATCH v2 23/28] binder: document the final page calculation

On Mon, Dec 4, 2023 at 3:39 PM Carlos Llamas <cmllamas@...gle.com> wrote:
>
> On Mon, Dec 04, 2023 at 11:57:27AM +0000, Alice Ryhl wrote:
> > > The code to determine the page range for binder_lru_freelist_del() is
> > > quite obscure. It leverages the buffer_size calculated before doing an
> > > oversized buffer split. This is used to figure out if the last page is
> > > being shared with another active buffer. If so, the page gets trimmed
> > > out of the range as it has been previously removed from the freelist.
> > >
> > > This would be equivalent to getting the start page of the next in-use
> > > buffer explicitly. However, the code for this is much larger as we can
> > > see in binder_free_buf_locked() routine. Instead, lets settle on
> > > documenting the tricky step and using better names for now.
> > >
> > > I believe an ideal solution would be to count the binder_page->users to
> > > determine when a page should be added or removed from the freelist.
> > > However, this is a much bigger change than what I'm willing to risk at
> > > this time.
> > >
> > > Signed-off-by: Carlos Llamas <cmllamas@...gle.com>
> >
> > Yes, this does help somewhat.
> >
> > However, `curr_last_page` is actually not the last page. It's the last
> > page plus one, since `binder_lru_freelist_del` is exclusive on this
> > argument. Maybe rename it to `curr_after_last_page` or something like
> > that? Or maybe even just `curr_last_page_plus_one`.
>
> hmmm, I don't know. I think this could be more confusing, the plus-one
> is only because of the way that binder_lru_freelist_del() processes the
> final page. So you could interpret the name both ways. Do we _really_
> need the extra comments to make it clear?
>
> This solution is too complex anyway, it should really be replaced with a
> binder_page->nr_users to determine when to add/remove from the lru.

You could also just remove the `next_used_page` part entirely. This
means that you will sometimes call `binder_lru_freelist_del` on a page
that's in use, but that's just a no-op.

Alice