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

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Message-ID: <8586b19c-2e14-4164-888f-8c3b86f3f963@redhat.com>
Date: Thu, 11 Jul 2024 20:24:07 +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
Subject: Re: [PATCH v22 1/4] mm: add MAP_DROPPABLE for designating always
 lazily freeable mappings

On 11.07.24 20:08, Jason A. Donenfeld wrote:
> On Thu, Jul 11, 2024 at 07:56:39PM +0200, Jason A. Donenfeld wrote:
>> On Thu, Jul 11, 2024 at 07:54:34PM +0200, Jason A. Donenfeld wrote:
>>> On Thu, Jul 11, 2024 at 07:27:27PM +0200, David Hildenbrand wrote:
>>>>> 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.
>>>
>>> I was thinking that too, but I'm afraid it's going to be another
>>> whack-a-mole nightmare. Even for things like task_mmu in procfs that
>>> show stats, that's going to be wonky.
>>>
>>> Any other flags we can overload that aren't going to be already used in
>>> our case?
>>
>> PG_error / folio_set_error seems unused in the non-IO case.
> 

Note that Willy is about to remove PG_error IIRC.

> And PG_large_rmappable seems to only be used for hugetlb branches.

It should be set for THP/large folios.

-- 
Cheers,

David / dhildenb