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

Alistair Popple
2021-03-12T04:43:53Z

On Tuesday, 9 March 2021 11:49:49 PM AEDT Matthew Wilcox wrote:
> 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():

I am happy to clean this up at the same time. It did seem a odd when I added
it and I had meant to follow up on it some more.

> 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); }

Thanks for expanding that out, makes sense and matches my reading as well.
Will remove the redundant compound_head() call in PageLocked() for the next
revision.

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