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

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Message-ID: <20250902091810.4854-1-roypat@amazon.co.uk>
Date: Tue, 2 Sep 2025 09:18:12 +0000
From: "Roy, Patrick" <roypat@...zon.co.uk>
To: "tabba@...gle.com" <tabba@...gle.com>
CC: "ackerleytng@...gle.com" <ackerleytng@...gle.com>, "david@...hat.com"
	<david@...hat.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>, "Roy, Patrick"
	<roypat@...zon.co.uk>, "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 Tue, 2025-09-02 at 09:50 +0100, Fuad Tabba wrote:
> On Tue, 2 Sept 2025 at 09:46, David Hildenbrand <david@...hat.com> wrote:
>>
>> 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).
> 
> Not for the scenario I'm worried about. The compiler didn't complain
> about this (from this patch):
> 
> +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);
> +}
> 
> vma_is_no_direct_map() takes a const, but mapping_no_direct_map()
> doesn't. For now, mapping_no_direct_map() doesn't modify anything. But
> it could, and the compiler wouldn't complain.

Wouldn't this only be a problem if vma->vm_file->f_mapping was a 'const struct
address_space *const'? I thought const-ness doesn't leak into pointers (e.g.
even above, vma_is_no_direct_map isn't allowed to make vma point at something
else, but it could modify the pointed-to vm_area_struct).

> Cheers,
> /fuad
> 
> 
>> Agreed that we should be using const * for these simple getter/test
>> functions.
>>
>> --
>> Cheers
>>
>> David / dhildenb
>>