Re: [PATCHv4 03/24] mm: avoid PG_locked on tail pages

[Christoph Lameter]

On Wed, 4 Mar 2015, Kirill A. Shutemov wrote:

> index c851ff92d5b3..58b98bced299 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -207,7 +207,8 @@ static inline int __TestClearPage##uname(struct page *page) { return 0; }
>
>  struct page;	/* forward declaration */
>
> -TESTPAGEFLAG(Locked, locked)
> +#define PageLocked(page) test_bit(PG_locked, &compound_head(page)->flags)
> +
>  PAGEFLAG(Error, error) TESTCLEARFLAG(Error, error)

Hmmm... Now one of the pageflag functions operates on the head page unlike
the other pageflag functions that only operate on the flag indicated.

Given that pageflags provide a way to implement checks for head / tail
pages this seems to be a bad idea.
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