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

[David Hildenbrand]

On 09.08.22 10:45, Huang, Ying wrote:
> Peter Xu <peterx@xxxxxxxxxx> 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@xxxxxxxxxx> 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.

We could max_swapfile_size()->__max_swapfile_size()

and then simply have a new max_swapfile_size() that caches the value in
a static variable. If that turns out to be worth the trouble.

-- 
Thanks,

David / dhildenb