On Wed, Mar 06, 2024 at 10:24:18AM -0800, Suren Baghdasaryan wrote:
> When a non-compound multi-order page is freed, it is possible that a
> speculative reference keeps the page pinned. In this case we free all
> pages except for the first page, which will be freed later by the last
> put_page(). However put_page() ignores the order of the page being freed,
> treating it as a 0-order page. This creates a memory accounting imbalance
> because the pages freed in __free_pages() do not have their own alloc_tag
> and their memory was accounted to the first page. To fix this the first
> page should adjust its allocation size counter when "tail" pages are freed.
It's not "ignored". It's not available!
Better wording:
However the page passed to put_page() is indisinguishable from an
order-0 page, so it cannot do the accounting, just as it cannot free
the subsequent pages. Do the accounting here, where we free the pages.
(I'm sure further improvements are possible)
> +static inline void pgalloc_tag_sub_bytes(struct alloc_tag *tag, unsigned int order)
> +{
> + if (mem_alloc_profiling_enabled() && tag)
> + this_cpu_sub(tag->counters->bytes, PAGE_SIZE << order);
> +}
This is a terribly named function. And it's not even good for what we
want to use it for.
static inline void pgalloc_tag_sub_pages(struct alloc_tag *tag, unsigned int nr)
{
if (mem_alloc_profiling_enabled() && tag)
this_cpu_sub(tag->counters->bytes, PAGE_SIZE * nr);
}
> +++ b/mm/page_alloc.c
> @@ -4697,12 +4697,21 @@ void __free_pages(struct page *page, unsigned int order)
> {
> /* get PageHead before we drop reference */
> int head = PageHead(page);
> + struct alloc_tag *tag = pgalloc_tag_get(page);
>
> if (put_page_testzero(page))
> free_the_page(page, order);
> else if (!head)
> - while (order-- > 0)
> + while (order-- > 0) {
> free_the_page(page + (1 << order), order);
> + /*
> + * non-compound multi-order page accounts all allocations
> + * to the first page (just like compound one), therefore
> + * we need to adjust the allocation size of the first
> + * page as its order is ignored when put_page() frees it.
> + */
> + pgalloc_tag_sub_bytes(tag, order);
- else if (!head
+ else if (!head) {
+ pgalloc_tag_sub_pages(1 << order - 1);
while (order-- > 0)
free_the_page(page + (1 << order), order);
+ }
It doesn't need a comment, it's obvious what you're doing.
