linux-kernel - Re: [RFC PATCH] mm: avoid clearing user movable page twice with init_on

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Message-ID: <b8272cb4-aee8-45ad-8dff-353444b3fa74@redhat.com>
Date: Tue, 8 Oct 2024 15:06:24 +0200
From: David Hildenbrand <david@...hat.com>
To: Vlastimil Babka <vbabka@...e.cz>, Zi Yan <ziy@...dia.com>
Cc: linux-mm@...ck.org, Alexander Potapenko <glider@...gle.com>,
 Kees Cook <keescook@...omium.org>, Andrew Morton
 <akpm@...ux-foundation.org>, "Matthew Wilcox (Oracle)"
 <willy@...radead.org>, Miaohe Lin <linmiaohe@...wei.com>,
 Kefeng Wang <wangkefeng.wang@...wei.com>, John Hubbard
 <jhubbard@...dia.com>, "Huang, Ying" <ying.huang@...el.com>,
 linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] mm: avoid clearing user movable page twice with
 init_on_alloc=1

On 08.10.24 14:57, Vlastimil Babka wrote:
> On 10/8/24 13:52, Zi Yan wrote:
>> On 8 Oct 2024, at 4:26, David Hildenbrand wrote:
>>
>>>
>>> I remember we discussed that in the past and that we do *not* want to sprinkle these CONFIG_INIT_ON_ALLOC_DEFAULT_ON checks all over the kernel.
>>>
>>> Ideally, we'd use GFP_ZERO and have the buddy just do that for us? There is the slight chance that we zero-out when we're not going to use the allocated folio, but ... that can happen either way even with the current code?
>>
>> I agree that putting CONFIG_INIT_ON_ALLOC_DEFAULT_ON here is not ideal, but
> 
> Create some nice inline wrapper for the test and it will look less ugly? :)
> 
>> folio_zero_user() uses vmf->address to improve cache performance by changing
>> subpage clearing order. See commit c79b57e462b5 ("mm: hugetlb: clear target
>> sub-page last when clearing huge page”). If we use GFP_ZERO, we lose this
>> optimization. To keep it, vmf->address will need to be passed to allocation
>> code. Maybe that is acceptable?
> 
> I'd rather not change the page allocation code for this...

Although I'm curious if that optimization from 2017 is still valuable :)

-- 
Cheers,

David / dhildenb