Re: [PATCH v5 1/8] mm: Remove special swap entry functions

[Matthew Wilcox <willy@infradead.org>]

On Tue, Mar 09, 2021 at 11:14:58PM +1100, Alistair Popple wrote:
> -static inline struct page *migration_entry_to_page(swp_entry_t entry)
> -{
> -	struct page *p = pfn_to_page(swp_offset(entry));
> -	/*
> -	 * Any use of migration entries may only occur while the
> -	 * corresponding page is locked
> -	 */
> -	BUG_ON(!PageLocked(compound_head(p)));
> -	return p;
> -}

> +static inline struct page *pfn_swap_entry_to_page(swp_entry_t entry)
> +{
> +	struct page *p = pfn_to_page(swp_offset(entry));
> +
> +	/*
> +	 * Any use of migration entries may only occur while the
> +	 * corresponding page is locked
> +	 */
> +	BUG_ON(is_migration_entry(entry) && !PageLocked(compound_head(p)));
> +
> +	return p;
> +}

I appreciate you're only moving this code, but PageLocked includes an
implicit compound_head():

1. __PAGEFLAG(Locked, locked, PF_NO_TAIL)

2. #define __PAGEFLAG(uname, lname, policy)                                \
        TESTPAGEFLAG(uname, lname, policy)                              \

3. #define TESTPAGEFLAG(uname, lname, policy)                              \
static __always_inline int Page##uname(struct page *page)               \
        { return test_bit(PG_##lname, &policy(page, 0)->flags); }

4. #define PF_NO_TAIL(page, enforce) ({                                    \
                VM_BUG_ON_PGFLAGS(enforce && PageTail(page), page);     \
                PF_POISONED_CHECK(compound_head(page)); })

5. #define PF_POISONED_CHECK(page) ({                                      \
                VM_BUG_ON_PGFLAGS(PagePoisoned(page), page);            \
                page; })


This macrology isn't easy to understand the first time you read it (nor,
indeed, the tenth time), so let me decode it:

Substitute 5 into 4 and remove irrelevancies:

6. #define PF_NO_TAIL(page, enforce) compound_head(page)

Expand 1 in 2:

7.	TESTPAGEFLAG(Locked, locked, PF_NO_TAIL)

Expand 7 in 3:

8. static __always_inline int PageLocked(struct page *page)
	{ return test_bit(PG_locked, &PF_NO_TAIL(page, 0)->flags); }

Expand 6 in 8:

9. static __always_inline int PageLocked(struct page *page)
	{ return test_bit(PG_locked, &compound_head(page)->flags); }

(in case it's not clear, compound_head() is idempotent.  that is:
	f(f(a)) == f(a))