Re: [PATCH] mm/page_alloc: make calling prep_compound_head more reliable

[Matthew Wilcox <willy@infradead.org>]

On Wed, Jun 08, 2022 at 08:17:35PM +0800, Miaohe Lin wrote:
> +++ b/mm/page_alloc.c
> @@ -6771,13 +6771,18 @@ static void __ref memmap_init_compound(struct page *head,
>                 set_page_count(page, 0);
> 
>                 /*
> -                * The first tail page stores compound_mapcount_ptr() and
> -                * compound_order() and the second tail page stores
> -                * compound_pincount_ptr(). Call prep_compound_head() after
> -                * the first and second tail pages have been initialized to
> -                * not have the data overwritten.
> +                * The first tail page stores compound_mapcount_ptr(),
> +                * compound_order() and compound_pincount_ptr(). Call
> +                * prep_compound_head() after the first tail page have
> +                * been initialized to not have the data overwritten.
> +                *
> +                * Note the idea to make this right after we initialize
> +                * the offending tail pages is trying to take advantage
> +                * of the likelihood of those tail struct pages being
> +                * cached given that we will read them right after in
> +                * prep_compound_head().

It's not that we'll read them again, it's that the cacheline will still
be in cache, and therefore dirty.

Honestly, I don't think we need this extra explanation in a comment.
Just change the first paragraph to reflect reality and leave it at that.

>                  */
> -               if (pfn == head_pfn + 2)
> +               if (unlikely(pfn == head_pfn + 1))

We definitely don't need the unlikely here.

>                         prep_compound_head(head, order);
>         }
>  }
> 
> Or am I miss something?
> 
> Thanks!
> 
> > .
> > 
>