linux-kernel - Re: [PATCH v21 1/4] mm: add VM_DROPPABLE for designating always lazily freeable mappings

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Message-ID: <6e978965-5bfd-483d-9504-f362fdf19ec5@redhat.com>
Date: Wed, 10 Jul 2024 05:53:12 +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 v21 1/4] mm: add VM_DROPPABLE for designating always
 lazily freeable mappings

On 10.07.24 05:34, Jason A. Donenfeld wrote:
> On Wed, Jul 10, 2024 at 05:05:06AM +0200, David Hildenbrand wrote:
>> On 09.07.24 04:17, Jason A. Donenfeld wrote:
>>> Hi David,
>>>
>>> On Mon, Jul 08, 2024 at 10:21:09PM +0200, David Hildenbrand wrote:
>>>> BTW, I was just trying to understand how MADV_FREE + MAP_DROPPABLE would
>>>> behave without any swap space around.
>>>>
>>>> Did you experiment with that?
>>>
>>> You mean on a system without any swap configured? That's actually my
>>> primary test environment for this. It behaves as expected: when ram
>>> fills up and the scanner is trying to reclaim what it can,
>>> folio_test_swapbacked(folio) is false, and the memory gets freed. After,
>>> reads fault in a zero page. So it's working as expected.
>>
>> Okay, just to be clear: no swap/zram/zswap. The reclaim code regarding
>> not scanning anonymous memory without swap was a bit confusing.
> 
> Right, no swap, as boring a system as can be. I've experimented with
> that behavior on my swap-less 64GB thinkpad, as well as on little
> special purpose VMs, where I hacked the VM_DROPPABLE test code into the
> wireguard test suite.

Great, thanks!

-- 
Cheers,

David / dhildenb