Re: [PATCH 1/1] mm/page_alloc: update comments in __free_pages_ok()

From: Andrew Morton
Date: Thu Dec 01 2022 - 17:42:24 EST


On Thu, 1 Dec 2022 21:50:45 +0800 wonder_rock@xxxxxxx wrote:

> Add a comment to explain why we call get_pfnblock_migratetype() twice
> in __free_pages_ok().
>
> ...
>
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1702,6 +1702,11 @@ static void __free_pages_ok(struct page *page, unsigned int order,
> if (!free_pages_prepare(page, order, true, fpi_flags))
> return;
>
> + /*
> + * Calling get_pfnblock_migratetype() without spin_lock_irqsave() here
> + * is used to avoid calling get_pfnblock_migratetype() under the lock.
> + * This will reduce the lock holding time.
> + */
> migratetype = get_pfnblock_migratetype(page, pfn);
>
> spin_lock_irqsave(&zone->lock, flags);

I guess that if the comment helped one reader, it will help others.
But this is a pretty common trick in MM and most readers will recognize
it.

That being said, get_pfnblock_migratetype() is pretty lightweight.
Particularly when compared with __free_one_page(). I wonder if the
optimization does much good.

If the second call to get_pfnblock_migratetype() is almost never performed
then we just added a bunch of testing and branching inside the lock
which actually made things worse!