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

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Message-ID: <98798483-dfcd-451e-94bb-57d830bf68d8@redhat.com>
Date: Thu, 11 Jul 2024 19:24:33 +0200
From: David Hildenbrand <david@...hat.com>
To: "Jason A. Donenfeld" <Jason@...c4.com>,
 Linus Torvalds <torvalds@...ux-foundation.org>
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, 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 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? :)

-- 
Cheers,

David / dhildenb