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

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Message-ID: <Zo9gXAlF-82_EYX1@zx2c4.com>
Date: Thu, 11 Jul 2024 06:32:28 +0200
From: "Jason A. Donenfeld" <Jason@...c4.com>
To: David Hildenbrand <david@...hat.com>
Cc: linux-kernel@...r.kernel.org, patches@...ts.linux.dev,
	tglx@...utronix.de, linux-crypto@...r.kernel.org,
	linux-api@...r.kernel.org, x86@...nel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Adhemerval Zanella Netto <adhemerval.zanella@...aro.org>,
	Carlos O'Donell <carlos@...hat.com>,
	Florian Weimer <fweimer@...hat.com>, Arnd Bergmann <arnd@...db.de>,
	Jann Horn <jannh@...gle.com>,
	Christian Brauner <brauner@...nel.org>,
	David Hildenbrand <dhildenb@...hat.com>, linux-mm@...ck.org
Subject: Re: [PATCH v22 1/4] mm: add MAP_DROPPABLE for designating always
 lazily freeable mappings

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.