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

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Message-ID: <Zo8q7ePlOearG481@zx2c4.com>
Date: Thu, 11 Jul 2024 02:44:29 +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

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.

What's your analysis of this like?

Jason