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

David Hildenbrand
2024-07-11T19:20:57Z

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;

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)?

--
Cheers,

David / dhildenb