Re: [PATCH v5 03/12] mm: introduce AS_NO_DIRECT_MAP

[Fuad Tabba <tabba@google.com>]

On Tue, 2 Sept 2025 at 10:18, Roy, Patrick <roypat@amazon.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@redhat.com> wrote:
> >>
> >> On 02.09.25 09:59, Fuad Tabba wrote:
> >>> Hi Patrick,
> >>>
> >>> On Mon, 1 Sept 2025 at 15:56, Roy, Patrick <roypat@amazon.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@amazon.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.

Cheers,
/fuad

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