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

David Hildenbrand
2024-07-11T19:18:10Z

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;

--
Cheers,

David / dhildenb