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

[Alice Ryhl]

> 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@xxxxxxxxxx>

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`.

Alice