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

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Message-ID: <da3ea234-d6dd-4809-b2f5-fbfedacb9748@redhat.com>
Date: Thu, 11 Jul 2024 21:53:48 +0200
From: David Hildenbrand <david@...hat.com>
To: Yu Zhao <yuzhao@...gle.com>
Cc: "Jason A. Donenfeld" <Jason@...c4.com>,
 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 11.07.24 21:49, Yu Zhao wrote:
> On Thu, Jul 11, 2024 at 1:20 PM David Hildenbrand <david@...hat.com> wrote:
>>
>> 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;
> 
> This is an optimization to avoid an unnecessary trip to
> shrink_folio_list(), so it's safe to delete the entire 'if' block, and
> that would be preferable than leaving a dangling 'if'.

Great, thanks.

> 
>> 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)?
> 
> Not sure what you mean: either rmap sees pte_dirty() and does
> folio_mark_dirty() and then folio_set_swapbacked(); or MGLRU does the
> same sequence, with the first two steps in walk_pte_range() and the
> last one here.

Let me rephrase:

Checking for lazyfree is

"folio_test_anon(folio) && !folio_test_swapbacked(folio)"

Testing for dirtied lazyfree is

"folio_test_anon(folio) && !folio_test_swapbacked(folio) &&
  folio_test)dirty(folio)"

So I'm wondering about the missing folio_test_swapbacked() test.

-- 
Cheers,

David / dhildenb