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

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Message-ID: <1c8632b4-06a5-49da-be0c-6fc7ac2b3257@redhat.com>
Date: Thu, 11 Jul 2024 21:18:03 +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 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;

-- 
Cheers,

David / dhildenb