Re: [RFC PATCH 0/9] introduce PGTY_mgt_entry page_type

[Huan Yang <link@vivo.com>]

在 2025/7/24 17:45, Lorenzo Stoakes 写道:
> On Thu, Jul 24, 2025 at 05:36:27PM +0800, Huan Yang wrote:
>> 在 2025/7/24 17:32, David Hildenbrand 写道:
>>> On 24.07.25 11:20, David Hildenbrand wrote:
>>>> On 24.07.25 11:12, David Hildenbrand wrote:
>>>>> On 24.07.25 11:09, Huan Yang wrote:
>>>>>> 在 2025/7/24 16:59, David Hildenbrand 写道:
>>>>>>> On 24.07.25 10:44, Huan Yang wrote:
>>>>>>>> Summary
>>>>>>>> ==
>>>>>>>> This patchset reuses page_type to store migrate
>>>>>>>> entry count during the
>>>>>>>> period from migrate entry setup to removal, enabling accelerated VMA
>>>>>>>> traversal when removing migrate entries, following a similar
>>>>>>>> principle to
>>>>>>>> early termination when folio is unmapped in try_to_migrate.
>>>>>>> I absolutely detest (ab)using page types for that, so no from my side
>>>>>>> unless I am missing something important.
>>>>>>>
>>>>>>>> In my self-constructed test scenario, the migration
>>>>>>>> time can be reduced
>>>>>>> How relevant is that in practice?
>>>>>> IMO, any folio mapped < nr vma in mapping(anon_vma, addresss_space),
>>>>>> will benefit from this.
>>>>>>
>>>>>> So, all pages that have been COW-ed by child processes can be skipped.
>>>>> For small anon folios, you could use the anon-exclusive marker
>>>>> to derive
>>>>> "there can only be a single mapping".
>>>>>
>>>>> It's stored alongside the migration entry.
>>>>>
>>>>> So once you restored that single migration entry, you can just stop the
>>>>> walk.
>>>> Essentially, something (untested) like this:
>>>>
>>>> diff --git a/mm/migrate.c b/mm/migrate.c
>>>> index 425401b2d4e14..aa5bf96b1daee 100644
>>>> --- a/mm/migrate.c
>>>> +++ b/mm/migrate.c
>>>> @@ -421,6 +421,15 @@ static bool remove_migration_pte(struct folio
>>>> *folio,
>>>>                       /* No need to invalidate - it was non-present
>>>> before */
>>>>                    update_mmu_cache(vma, pvmw.address, pvmw.pte);
>>>> +
>>>> +               /*
>>>> +                * If the small anon folio is exclusive, here can be
>>>> exactly one
>>>> +                * page mapping -- the one we just restored.
>>>> +                */
>>>> +               if (!folio_test_large(folio) && (rmap_flags &
>>>> RMAP_EXCLUSIVE)) {
>>>> +                       page_vma_mapped_walk_done(&pvmw);
>>>> +                       break;
>>>> +               }
>>>>            }
>>>>               return true;
>>> Probably that won't really help I assume, because __folio_set_anon()
>>> will move the new anon folio under vma->anon_vma, not
>>> vma->anon_vma->root.
>>>
>>> So I assume you mean that we had a COW-shared folio now mapped only into
>>> some VMAs (some mappings in other processes removed due to CoW or
>>> similar).
>>>
>>> In that case aborting early can help.
>>>
>>> Not in all cases though, just imagine that the very last VMA we're
>>> iterating maps the page. You have to iterate through all of them either
>>> way ... no way around that, really.
>> Indeed, whether we can exit the loop early depends on the position of the
>> terminating VMA in the tree.
>>
>> I think a better approach would be to remove the fully COW-ed VMAs and their
>> associated AVCs from the anon_vma's tree.
>>
>> I've been researching this aspect, but haven't made any progress yet.(I have
>> some ideas, but the specific implementation is still challenging.)
>>
> Please leave this alone, I'm in the midst of trying to make fundamental changes
> to the anon rmap logic and it's really very subtle and indeed challenging (as
> you've seen).
OK, but I still try to research it, not only for upstream or something.
>
> Since I intend to change the whole mechanism around this, efforts to adjust the
> existing behaviour are going to strictly conflict with that.
>
> We are 'lazy' in actually properly accounting for fully CoW'd VMAs and so can
> only know 'maybe' if it has, I mean as from above you've noticed.
>
> The CoW hierarchy also makes life challenging, see vma_had_uncowed_parents() for
> an example of the subtlty.
>
> Having looked at anon rmap in detail, I have come to think the only sensible way
> forward is something fairly bold.

Woo, that's really interesting. I'm looking forward to this.

Thanks.


>
> Thanks!
>