linux-kernel - Re: [PATCH v5 03/12] mm: introduce AS_NO_DIRECT

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Message-ID: <862475d8-5a4f-44c3-9b3f-56319f70192d@redhat.com>
Date: Tue, 2 Sep 2025 10:46:52 +0200
From: David Hildenbrand <david@...hat.com>
To: Fuad Tabba <tabba@...gle.com>, "Roy, Patrick" <roypat@...zon.co.uk>
Cc: "ackerleytng@...gle.com" <ackerleytng@...gle.com>,
 "Manwaring, Derek" <derekmn@...zon.com>, "Thomson, Jack"
 <jackabt@...zon.co.uk>, "Kalyazin, Nikita" <kalyazin@...zon.co.uk>,
 "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
 "kvmarm@...ts.linux.dev" <kvmarm@...ts.linux.dev>,
 "linux-arm-kernel@...ts.infradead.org"
 <linux-arm-kernel@...ts.infradead.org>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "linux-mm@...ck.org" <linux-mm@...ck.org>,
 "pbonzini@...hat.com" <pbonzini@...hat.com>,
 "rppt@...nel.org" <rppt@...nel.org>, "seanjc@...gle.com"
 <seanjc@...gle.com>, "vbabka@...e.cz" <vbabka@...e.cz>,
 "will@...nel.org" <will@...nel.org>, "Cali, Marco" <xmarcalx@...zon.co.uk>
Subject: Re: [PATCH v5 03/12] mm: introduce AS_NO_DIRECT_MAP

On 02.09.25 09:59, Fuad Tabba wrote:
> Hi Patrick,
> 
> On Mon, 1 Sept 2025 at 15:56, Roy, Patrick <roypat@...zon.co.uk> wrote:
>>
>> On Mon, 2025-09-01 at 14:54 +0100, "Roy, Patrick" wrote:
>>>
>>> Hi Fuad!
>>>
>>> On Thu, 2025-08-28 at 11:21 +0100, Fuad Tabba wrote:
>>>> Hi Patrick,
>>>>
>>>> On Thu, 28 Aug 2025 at 10:39, Roy, Patrick <roypat@...zon.co.uk> wrote:
>>>>> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
>>>>> index 12a12dae727d..b52b28ae4636 100644
>>>>> --- a/include/linux/pagemap.h
>>>>> +++ b/include/linux/pagemap.h
>>>>> @@ -211,6 +211,7 @@ enum mapping_flags {
>>>>>                                     folio contents */
>>>>>          AS_INACCESSIBLE = 8,    /* Do not attempt direct R/W access to the mapping */
>>>>>          AS_WRITEBACK_MAY_DEADLOCK_ON_RECLAIM = 9,
>>>>> +       AS_NO_DIRECT_MAP = 10,  /* Folios in the mapping are not in the direct map */
>>>>>          /* Bits 16-25 are used for FOLIO_ORDER */
>>>>>          AS_FOLIO_ORDER_BITS = 5,
>>>>>          AS_FOLIO_ORDER_MIN = 16,
>>>>> @@ -346,6 +347,21 @@ static inline bool mapping_writeback_may_deadlock_on_reclaim(struct address_spac
>>>>>          return test_bit(AS_WRITEBACK_MAY_DEADLOCK_ON_RECLAIM, &mapping->flags);
>>>>>   }
>>>>>
>>>>> +static inline void mapping_set_no_direct_map(struct address_space *mapping)
>>>>> +{
>>>>> +       set_bit(AS_NO_DIRECT_MAP, &mapping->flags);
>>>>> +}
>>>>> +
>>>>> +static inline bool mapping_no_direct_map(struct address_space *mapping)
>>>>> +{
>>>>> +       return test_bit(AS_NO_DIRECT_MAP, &mapping->flags);
>>>>> +}
>>>>> +
>>>>> +static inline bool vma_is_no_direct_map(const struct vm_area_struct *vma)
>>>>> +{
>>>>> +       return vma->vm_file && mapping_no_direct_map(vma->vm_file->f_mapping);
>>>>> +}
>>>>> +
>>>> Any reason vma is const whereas mapping in the function that it calls
>>>> (defined above it) isn't?
>>>
>>> Ah, I cannot say that that was a conscious decision, but rather an artifact of
>>> the code that I looked at for reference when writing these two simply did it
>>> this way.  Are you saying both should be const, or neither (in my mind, both
>>> could be const, but the mapping_*() family of functions further up in this file
>>> dont take const arguments, so I'm a bit unsure now)?
>>
>> Hah, just saw
>> https://lore.kernel.org/linux-mm/20250901123028.3383461-3-max.kellermann@ionos.com/.
>> Guess that means "both should be const" then :D
> 
> I don't have any strong preference regarding which way, as long as
> it's consistent. The thing that should be avoided is having one
> function with a parameter marked as const, pass that parameter (or
> something derived from it), to a non-const function. 

I think the compiler will tell you that that is not ok (and you'd have 
to force-cast the const it away).

Agreed that we should be using const * for these simple getter/test 
functions.

-- 
Cheers

David / dhildenb