On 12/10/24 03:39, Alexei Starovoitov wrote:
> From: Alexei Starovoitov <ast@xxxxxxxxxx>
>
> Introduce free_pages_nolock() that can free a page without taking locks.
> It relies on trylock only and can be called from any context.
>
> Signed-off-by: Alexei Starovoitov <ast@xxxxxxxxxx>
<snip>
> +/* Can be called while holding raw_spin_lock or from IRQ. RCU must be watching. */
> +void free_pages_nolock(struct page *page, unsigned int order)
What does "RCU must be watching" mean and why?
> +{
> + int head = PageHead(page);
> + struct alloc_tag *tag = pgalloc_tag_get(page);
> +
> + if (put_page_testzero(page)) {
> + __free_unref_page(page, order, FPI_TRYLOCK);
> + } else if (!head) {
> + pgalloc_tag_sub_pages(tag, (1 << order) - 1);
> + while (order-- > 0)
> + __free_unref_page(page + (1 << order), order, FPI_TRYLOCK);
Not your fault for trying to support everything __free_pages did,
specifically order > 0 pages that are not compound and thus needing this
extra !head branch. We'd love to get rid of that eventually in
__free_pages(), but at least I think we don't need to support it in a new
API and instead any users switching to it should know it's not supported.
I suppose BFP doesn't need that, right?
Maybe if the function was taking a folio instead of page, it would be the
best as that has to be order-0 or compound by definition. It also wouldn't
need the order parameter. What do you think, Matthew?
> + }
> +}
> +
> void free_pages(unsigned long addr, unsigned int order)
> {
> if (addr != 0) {
