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

[Jason A. Donenfeld]

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.