Re: [PATCH v22 1/4] mm: add MAP_DROPPABLE for designating always lazily freeable mappings

David Hildenbrand
2024-07-11T19:53:53Z

On 11.07.24 21:49, Yu Zhao wrote:
> On Thu, Jul 11, 2024 at 1:20 PM David Hildenbrand <david@redhat.com> wrote:
>>
>> On 11.07.24 21:18, David Hildenbrand wrote:
>>> On 11.07.24 20:56, David Hildenbrand wrote:
>>>> On 11.07.24 20:54, Jason A. Donenfeld wrote:
>>>>> On Thu, Jul 11, 2024 at 08:24:07PM +0200, David Hildenbrand wrote:
>>>>>>> And PG_large_rmappable seems to only be used for hugetlb branches.
>>>>>>
>>>>>> It should be set for THP/large folios.
>>>>>
>>>>> And it's tested too, apparently.
>>>>>
>>>>> Okay, well, how disappointing is this below? Because I'm running out of
>>>>> tricks for flag reuse.
>>>>>
>>>>> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
>>>>> index b9e914e1face..c1ea49a7f198 100644
>>>>> --- a/include/linux/page-flags.h
>>>>> +++ b/include/linux/page-flags.h
>>>>> @@ -110,6 +110,7 @@ enum pageflags {
>>>>> PG_workingset,
>>>>> PG_error,
>>>>> PG_owner_priv_1, /* Owner use. If pagecache, fs may use*/
>>>>> + PG_owner_priv_2,
>>>>
>>>> Oh no, no new page flags please :)
>>>>
>>>> Maybe just follow what Linux suggested: pass vma to pte_dirty() and
>>>> always return false for these special VMAs.
>>>
>>> ... or look into removing that one case that gives us headake.
>>>
>>> No idea what would happen if we do the following:
>>>
>>> CCing Yu Zhao.
>>>
>>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>>> index 0761f91b407f..d1dfbd4fd38d 100644
>>> --- a/mm/vmscan.c
>>> +++ b/mm/vmscan.c
>>> @@ -4280,14 +4280,9 @@ static bool sort_folio(struct lruvec *lruvec, struct folio *folio, struct scan_c
>>> return true;
>>> }
>>>
>>> - /* dirty lazyfree */
>>> - if (type == LRU_GEN_FILE && folio_test_anon(folio) && folio_test_dirty(folio)) {
>>> - success = lru_gen_del_folio(lruvec, folio, true);
>>> - VM_WARN_ON_ONCE_FOLIO(!success, folio);
>>> - folio_set_swapbacked(folio);
>>> - lruvec_add_folio_tail(lruvec, folio);
>>> - return true;
>>> - }
>>> + /* lazyfree: we may not be allowed to set swapbacked: MAP_DROPPABLE */
>>> + if (type == LRU_GEN_FILE && folio_test_anon(folio) && folio_test_dirty(folio))
>>> + return false;
>
> This is an optimization to avoid an unnecessary trip to
> shrink_folio_list(), so it's safe to delete the entire 'if' block, and
> that would be preferable than leaving a dangling 'if'.

Great, thanks.

>
>> Note that something is unclear to me: are we maybe running into that
>> code also if folio_set_swapbacked() is already set and we are not in the
>> lazyfree path (in contrast to what is documented)?
>
> Not sure what you mean: either rmap sees pte_dirty() and does
> folio_mark_dirty() and then folio_set_swapbacked(); or MGLRU does the
> same sequence, with the first two steps in walk_pte_range() and the
> last one here.

Let me rephrase:

Checking for lazyfree is

"folio_test_anon(folio) && !folio_test_swapbacked(folio)"

Testing for dirtied lazyfree is

"folio_test_anon(folio) && !folio_test_swapbacked(folio) &&
folio_test)dirty(folio)"

So I'm wondering about the missing folio_test_swapbacked() test.

--
Cheers,

David / dhildenb