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

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Message-ID: <2c464271-1c61-4cd8-bd4e-4bd8aa01fa00@redhat.com>
Date: Thu, 11 Jul 2024 21:20:48 +0200
From: David Hildenbrand <david@...hat.com>
To: "Jason A. Donenfeld" <Jason@...c4.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,
 Yu Zhao <yuzhao@...gle.com>
Subject: Re: [PATCH v22 1/4] mm: add MAP_DROPPABLE for designating always
 lazily freeable mappings

On 11.07.24 21:18, David Hildenbrand wrote:
> On 11.07.24 20:56, David Hildenbrand wrote:
>> On 11.07.24 20:54, Jason A. Donenfeld wrote:
>>> On Thu, Jul 11, 2024 at 08:24:07PM +0200, David Hildenbrand wrote:
>>>>> And PG_large_rmappable seems to only be used for hugetlb branches.
>>>>
>>>> It should be set for THP/large folios.
>>>
>>> And it's tested too, apparently.
>>>
>>> Okay, well, how disappointing is this below? Because I'm running out of
>>> tricks for flag reuse.
>>>
>>> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
>>> index b9e914e1face..c1ea49a7f198 100644
>>> --- a/include/linux/page-flags.h
>>> +++ b/include/linux/page-flags.h
>>> @@ -110,6 +110,7 @@ enum pageflags {
>>>     	PG_workingset,
>>>     	PG_error,
>>>     	PG_owner_priv_1,	/* Owner use. If pagecache, fs may use*/
>>> +	PG_owner_priv_2,
>>
>> Oh no, no new page flags please :)
>>
>> Maybe just follow what Linux suggested: pass vma to pte_dirty() and
>> always return false for these special VMAs.
> 
> ... or look into removing that one case that gives us headake.
> 
> No idea what would happen if we do the following:
> 
> CCing Yu Zhao.
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 0761f91b407f..d1dfbd4fd38d 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -4280,14 +4280,9 @@ static bool sort_folio(struct lruvec *lruvec, struct folio *folio, struct scan_c
>                   return true;
>           }
>    
> -       /* 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;
> -       }
> +       /* lazyfree: we may not be allowed to set swapbacked: MAP_DROPPABLE */
> +       if (type == LRU_GEN_FILE && folio_test_anon(folio) && folio_test_dirty(folio))
> +               return false;

Note that something is unclear to me: are we maybe running into that 
code also if folio_set_swapbacked() is already set and we are not in the 
lazyfree path (in contrast to what is documented)?

-- 
Cheers,

David / dhildenb