Re: [PATCH] mm/migrate_device.c: Fix a misleading and out-dated comment

[David Hildenbrand <david@redhat.com>]

On 26.08.22 17:03, Peter Xu wrote:
> On Fri, Aug 26, 2022 at 10:34:15AM +1000, Alistair Popple wrote:
>>
>> Peter Xu <peterx@redhat.com> writes:
>>
>>> On Thu, Aug 25, 2022 at 11:49:05AM +1000, Alistair Popple wrote:
>>>> Commit ab09243aa95a ("mm/migrate.c: remove MIGRATE_PFN_LOCKED") changed
>>>> the way trylock_page() in migrate_vma_collect_pmd() works without
>>>> updating the comment. Reword the comment to be less misleading and a
>>>> better reflection of what happens.
>>>>
>>>> Signed-off-by: Alistair Popple <apopple@nvidia.com>
>>>> Reported-by: Peter Xu <peterx@redhat.com>
>>>> Fixes: ab09243aa95a ("mm/migrate.c: remove MIGRATE_PFN_LOCKED")
>>>> ---
>>>>  mm/migrate_device.c | 8 +++++---
>>>>  1 file changed, 5 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/mm/migrate_device.c b/mm/migrate_device.c
>>>> index 5052093d0262..0736f846de0b 100644
>>>> --- a/mm/migrate_device.c
>>>> +++ b/mm/migrate_device.c
>>>> @@ -179,9 +179,11 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
>>>>  		get_page(page);
>>>>
>>>>  		/*
>>>> -		 * Optimize for the common case where page is only mapped once
>>>> -		 * in one process. If we can lock the page, then we can safely
>>>> -		 * set up a special migration page table entry now.
>>>> +		 * If we can't lock the page we can't migrate it. If we can it's
>>>> +		 * safe to set up a migration entry now. In the common case
>>>> +		 * where the page is mapped once in a single process setting up
>>>> +		 * the migration entry now is an optimisation to avoid walking
>>>> +		 * the rmap later with try_to_migrate().
>>>>  		 */
>>>
>>> IMHO the last sentence can still be a bit confusing - here we 100% rely on
>>> the trylock() to proceed or we'll stop migration right away. IMHO that
>>> means this is not an optimization, since optimizations should always be
>>> optional but not the case here.
>>
>> We have to lock the page here, we don't have to install the migration
>> entries. Installing the migration entries here is optional and is the
>> optimisation.
> 
> I see what you mean now.
> 
>>
>>> Meanwhile it'll be great to also mention about why trylock is needed and no
>>> further attempt to use lock_page().  The comment in prepare() previously
>>> was great but unfortunately that code clip was removed.
>>
>> Will add.
>>
>>> In short, do you think something like this might be clearer?
>>
>> I think it's important to mention the optimisation, otherwise the
>> temptation is to remove the installation of migration entries here and
>> rely on try_to_migrate() to do it later. I would actually like to be
>> able to do that because it simplifies the code in many ways but based on
>> my testing the optimisation turns out to be very worth while.
>>
>>> 		/*
>>> 		 * We rely on the trylock() to migrate the pte.  If this
>>> 		 * fails, we'll fail the migration of this page.  IOW, the
>>> 		 * migration is very much best-effort, just like we'll also
>>> 		 * bail out if we found page pinned by other users after
>>> 		 * page being locked.
>>
>> Honestly I think this describes what the code does rather than why and
>> is likely to become outdated and confusing. IMHO it's quite clear from
>> the code that the migration will fail here if we can't lock the page.
> 
> If you see that's what I was struggling to understand previously, so not
> clear at least to me. :) Since normally a function like page migration
> should (from the gut feeling) not rely on trylock only.

IIRC, ordinary page migration will also only trylock. See
isolate_movable_page().


-- 
Thanks,

David / dhildenb