Re: [PATCH v2 2/2] mm: Remember young/dirty bit for page migrations

["Huang, Ying" <ying.huang@intel.com>]

Peter Xu <peterx@redhat.com> writes:

> On Thu, Aug 04, 2022 at 03:40:57PM -0700, Nadav Amit wrote:
>> On Aug 4, 2022, at 1:39 PM, Peter Xu <peterx@redhat.com> wrote:
>> > +
>> > static inline bool is_pfn_swap_entry(swp_entry_t entry);
>> > 
>> > /* Clear all flags but only keep swp_entry_t related information */
>> > @@ -265,6 +285,57 @@ static inline swp_entry_t make_writable_migration_entry(pgoff_t offset)
>> > 	return swp_entry(SWP_MIGRATION_WRITE, offset);
>> > }
>> > 
>> > +/*
>> > + * Returns whether the host has large enough swap offset field to support
>> > + * carrying over pgtable A/D bits for page migrations.  The result is
>> > + * pretty much arch specific.
>> > + */
>> > +static inline bool migration_entry_supports_ad(void)
>> > +{
>> > +	/*
>> > +	 * max_swapfile_size() returns the max supported swp-offset plus 1.
>> > +	 * We can support the migration A/D bits iff the pfn swap entry has
>> > +	 * the offset large enough to cover all of them (PFN, A & D bits).
>> > +	 */
>> > +#ifdef CONFIG_SWAP
>> > +	return max_swapfile_size() >= (1UL << SWP_MIG_TOTAL_BITS);
>> 
>> This is an actual a function call (unless LTO has some trick). A bit of a
>> shame it cannot be at least memoized.
>> 
>> Or at least mark max_swapfile_size() as __attribute_const__ so it would not
>> be called twice for make_migration_entry_young() and
>> make_migration_entry_dirty().
>
> I didn't take too much effort on this one since we're on swap path and I
> assumed that's not a super hot path.  But __attribute_const__ sounds good
> and easy to get, thanks.
>
> Perhaps I should mark it on migration_entry_supports_ad() as a whole?  Note
> that unfortunately SWP_MIG_TOTAL_BITS may not be a const either (see how
> that define roots back to MAX_PHYSMEM_BITS, where on x86_64 it needs to
> check 5-lvl).

I think it's possible to memorize max_swapfile_size() or
migration_entry_supports_ad().  Although they are not constant, they are
not changed after initialized.  The challenge is to find a clean way to
initialize it.

Best Regards,
Huang, Ying