On 27/04/2023 22.53, Toke Høiland-Jørgensen wrote:
+noinline
static void page_pool_empty_ring(struct page_pool *pool)
{
struct page *page;
@@ -796,39 +828,29 @@ static void page_pool_scrub(struct page_pool *pool)
page_pool_empty_ring(pool);
}
So this is not in the diff context, but page_pool_empty_ring() does
this:
static void page_pool_empty_ring(struct page_pool *pool)
{
struct page *page;
/* Empty recycle ring */
while ((page = ptr_ring_consume_bh(&pool->ring))) {
/* Verify the refcnt invariant of cached pages */
if (!(page_ref_count(page) == 1))
pr_crit("%s() page_pool refcnt %d violation\n",
__func__, page_ref_count(page));
page_pool_return_page(pool, page);
}
}
...and with this patch, that page_pool_return_page() call will now free
the pool memory entirely when the last page is returned. When it does
this, the condition in the while loop will still execute afterwards; it
would return false, but if the pool was freed, it's now referencing
freed memory when trying to read from pool->ring.
Yes, that sounds like a problem.
So I think page_pool_empty_ring needs to either pull out all the pages
in the ring to an on-stack buffer before calling page_pool_return_page()
on them, or there needs to be some other way to break the loop early.
Let me address this one first, I'll get back to the other in another
reply. The usual/idiom way of doing this is to have a next pointer that
is populated inside the loop before freeing the object.
It should look like this (only compile tested):
static void page_pool_empty_ring(struct page_pool *pool)
{
struct page *page, *next;
next = ptr_ring_consume_bh(&pool->ring);
/* Empty recycle ring */
while (next) {
page = next;
next = ptr_ring_consume_bh(&pool->ring);
/* Verify the refcnt invariant of cached pages */
if (!(page_ref_count(page) == 1))
pr_crit("%s() page_pool refcnt %d violation\n",
__func__, page_ref_count(page));
page_pool_return_page(pool, page);
}
}
There are a couple of other places where page_pool_return_page() is
called in a loop where the loop variable lives inside struct page_pool,
so we need to be absolutely sure they will never be called in the
shutdown stage, or they'll have to be fixed as well.
The other loops are okay, but I spotted another problem in
__page_pool_put_page() in "Fallback/non-XDP mode", but that is fixable.
--Jesper
