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

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Message-ID: <6f045e34-8b88-49f7-957a-69130f6de949@redhat.com>
Date: Thu, 11 Jul 2024 22:59:06 +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 21:58, Yu Zhao wrote:
> On Thu, Jul 11, 2024 at 1:53 PM David Hildenbrand <david@...hat.com> wrote:
>>
>> 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.
> 
> It's not missing: type == LRU_GEN_FILE means folio_is_file_lru(),
> which in turn means !folio_test_swapbacked().
> 

Ahh, got it, thanks!

-- 
Cheers,

David / dhildenb