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

David Hildenbrand
2024-07-11T20:59:12Z

On 11.07.24 21:58, Yu Zhao wrote:
> On Thu, Jul 11, 2024 at 1:53 PM David Hildenbrand <david@redhat.com> wrote:
>>
>> 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.
>
> It's not missing: type == LRU_GEN_FILE means folio_is_file_lru(),
> which in turn means !folio_test_swapbacked().
>

Ahh, got it, thanks!

--
Cheers,

David / dhildenb