binder: document the final page calculation

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@google.com>
Link: https://lore.kernel.org/r/20231201172212.1813387-24-cmllamas@google.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/android/binder_alloc.c

@@ -446,8 +446,8 @@ static struct binder_buffer *binder_alloc_new_buf_locked(
 	struct rb_node *n = alloc->free_buffers.rb_node;
 	struct rb_node *best_fit = NULL;
 	struct binder_buffer *buffer;
-	unsigned long has_page_addr;
-	unsigned long end_page_addr;
+	unsigned long next_used_page;
+	unsigned long curr_last_page;
 	size_t buffer_size;
 
 	if (is_async && alloc->free_async_space < size) {
@@ -500,12 +500,16 @@ static struct binder_buffer *binder_alloc_new_buf_locked(
 		     "%d: binder_alloc_buf size %zd got buffer %pK size %zd\n",
 		      alloc->pid, size, buffer, buffer_size);
 
-	has_page_addr = (buffer->user_data + buffer_size) & PAGE_MASK;
-	end_page_addr = PAGE_ALIGN(buffer->user_data + size);
-	if (end_page_addr > has_page_addr)
-		end_page_addr = has_page_addr;
+	/*
+	 * Now we remove the pages from the freelist. A clever calculation
+	 * with buffer_size determines if the last page is shared with an
+	 * adjacent in-use buffer. In such case, the page has been already
+	 * removed from the freelist so we trim our range short.
+	 */
+	next_used_page = (buffer->user_data + buffer_size) & PAGE_MASK;
+	curr_last_page = PAGE_ALIGN(buffer->user_data + size);
 	binder_lru_freelist_del(alloc, PAGE_ALIGN(buffer->user_data),
-				end_page_addr);
+				min(next_used_page, curr_last_page));
 
 	rb_erase(&buffer->rb_node, &alloc->free_buffers);
 	buffer->free = 0;