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

[David Hildenbrand <david@redhat.com>]

On 11.07.24 06:32, Jason A. Donenfeld wrote:
> On Thu, Jul 11, 2024 at 02:44:29AM +0200, Jason A. Donenfeld wrote:
>> Hi David,
>>
>> On Wed, Jul 10, 2024 at 06:05:34AM +0200, David Hildenbrand wrote:
>>> BTW, do we have to handle the folio_set_swapbacked() in sort_folio() as well?
>>>
>>>
>>> 	/* 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;
>>> 	}
>>>
>>> Maybe more difficult because we don't have a VMA here ... hmm
>>>
>>> IIUC, we have to make sure that no folio_set_swapbacked() would ever get
>>> performed on these folios, correct?
>>
>> Hmmm, I'm trying to figure out what to do here, and if we have to do
>> something. All three conditions in that if statement will be true for a
>> folio in a droppable mapping. That's supposed to match MADV_FREE
>> mappings.
>>
>> What is the context of this, though? It's scanning pages for good ones
>> to evict into swap, right? So if it encounters one that's an MADV_FREE
>> page, it actually just wants to delete it, rather than sending it to
>> swap. So it looks like it does just that, and then sets the swapbacked
>> bit back to true, in case the folio is used for something differnet
>> later?
>>
>> If that's correct, then I don't think we need to do anything for this
>> one.
>>
>> If that's not correct, then we'll need to propagate the droppableness
>> to the folio level. But hopefully we don't need to do that.
> 
> Looks like that's not correct. This is for pages that have been dirtied
> since calling MADV_FREE. So, hm.
> 

Maybe we can find ways of simply never marking these pages dirty, so we 
don't have to special-case that code where we don't really have a VMA at 
hand?

-- 
Cheers,

David / dhildenb