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

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Message-ID: <ZpAbIW8vzEDsGvR0@zx2c4.com>
Date: Thu, 11 Jul 2024 19:49:21 +0200
From: "Jason A. Donenfeld" <Jason@...c4.com>
To: David Hildenbrand <david@...hat.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
	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,
	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 07:24:33PM +0200, David Hildenbrand wrote:
> On 11.07.24 19:17, Jason A. Donenfeld wrote:
> > On Thu, Jul 11, 2024 at 07:09:36PM +0200, Jason A. Donenfeld wrote:
> >> So, hmm... The swapbacked thing really seemed so simple... I wonder if
> >> there's a way of recovering that.
> > 
> > Not wanting to introduce a new bitflag, I went looking and noticed this:
> > 
> > /*
> >   * Private page markings that may be used by the filesystem that owns the page
> >   * for its own purposes.
> >   * - PG_private and PG_private_2 cause release_folio() and co to be invoked
> >   */
> > PAGEFLAG(Private, private, PF_ANY)
> > PAGEFLAG(Private2, private_2, PF_ANY) TESTSCFLAG(Private2, private_2, PF_ANY)
> > PAGEFLAG(OwnerPriv1, owner_priv_1, PF_ANY)
> >          TESTCLEARFLAG(OwnerPriv1, owner_priv_1, PF_ANY)
> > 
> > The below +4/-1 diff is pretty hacky and might be illegal in the state
> > of California, but I think it does work. The idea is that if that bit is
> > normally only used for filesystems, then in the anonymous case, it's
> > free to be used for this.
> > 
> > Any opinions about this, or a suggestion on how to do that in a less
> > ugly way?
> > 
> > Jason
> > 
> > 
> > diff --git a/mm/rmap.c b/mm/rmap.c
> > index 1f9b5a9cb121..090554277e4a 100644
> > --- a/mm/rmap.c
> > +++ b/mm/rmap.c
> > @@ -1403,6 +1403,8 @@ void folio_add_new_anon_rmap(struct folio *folio, struct vm_area_struct *vma,
> >   	 */
> >   	if (!(vma->vm_flags & VM_DROPPABLE))
> >   		__folio_set_swapbacked(folio);
> > +	else
> > +		folio_set_owner_priv_1(folio);
> 
> 
> PG_owner_priv_1 maps to PG_swapcache? :)

Oh, drat, it looks like this overloading is nothing new then.