Re: [PATCH 4/7] mm/hmm: properly handle migration pmd

["Zi Yan" <zi.yan@cs.rutgers.edu>]

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Hi Michal,

On 28 Aug 2018, at 11:45, Michal Hocko wrote:

> On Tue 28-08-18 17:42:06, Michal Hocko wrote:
>> On Tue 28-08-18 11:36:59, Jerome Glisse wrote:
>>> On Tue, Aug 28, 2018 at 05:24:14PM +0200, Michal Hocko wrote:
>>>> On Fri 24-08-18 20:05:46, Zi Yan wrote:
>>>> [...]
>>>>>> +	if (!pmd_present(pmd)) {
>>>>>> +		swp_entry_t entry = pmd_to_swp_entry(pmd);
>>>>>> +
>>>>>> +		if (is_migration_entry(entry)) {
>>>>>
>>>>> I think you should check thp_migration_supported() here, since PMD migration is only enabled in x86_64 systems.
>>>>> Other architectures should treat PMD migration entries as bad.
>>>>
>>>> How can we have a migration pmd entry when the migration is not
>>>> supported?
>>>
>>> Not sure i follow here, migration can happen anywhere (assuming
>>> that something like compaction is active or numa or ...). So this
>>> code can face pmd migration entry on architecture that support
>>> it. What is missing here is thp_migration_supported() call to
>>> protect the is_migration_entry() to avoid false positive on arch
>>> which do not support thp migration.
>>
>> I mean that architectures which do not support THP migration shouldn't
>> ever see any migration entry. So is_migration_entry should be always
>> false. Or do I miss something?
>
> And just to be clear. thp_migration_supported should be checked only
> when we actually _do_ the migration or evaluate migratability of the
> page. We definitely do want to sprinkle this check to all places where
> is_migration_entry is checked.

is_migration_entry() is a general check for swp_entry_t, so it can return
true even if THP migration is not enabled. is_pmd_migration_entry() always
returns false when THP migration is not enabled.

So the code can be changed in two ways, either replacing is_migration_entry()
with is_pmd_migration_entry() or adding thp_migration_supported() check
like Jerome did.

Does this clarify your question?

—
Best Regards,
Yan Zi

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