Message-ID: <8f571dc9-f2d4-46c2-a4b4-6854fa31da2e@redhat.com>
Date: Tue, 2 Sep 2025 11:54:59 +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"
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Subject: Re: [PATCH v5 03/12] mm: introduce AS_NO_DIRECT_MAP

On 02.09.25 11:21, Fuad Tabba wrote:
> On Tue, 2 Sept 2025 at 10:18, Roy, Patrick <roypat@...zon.co.uk> wrote:
>>
>> 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).
> 
> That's the thing, constness checks don't carry over to pointers within
> a struct, but a person reading the code would assume that a function
> with a parameter marked as const wouldn't modify anything related to
> that parameter.

Ah, thanks, I forgot that detail, it's only for embedded structs but not 
pointers.

I wonder if something (sparse?) could detect such cases.

-- 
Cheers

David / dhildenb

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