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

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Message-ID: <54b6de32-f127-4928-9f4a-acb8653e5c81@redhat.com>
Date: Thu, 11 Jul 2024 19:27:27 +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 19:24, 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? :)

Maybe the combination !swapbacked && swapcache could be used to indicate 
such folios. (we will never set swapbacked)

But likely we have to be a bit careful here. We don't want 
folio_test_swapcache() to return for folios that ... are not in the 
swapcache.

-- 
Cheers,

David / dhildenb