On Tue, 2025-09-02 at 09:50 +0100, Fuad Tabba wrote:

On Tue, 2 Sept 2025 at 09:46, David Hildenbrand [off-list ref] wrote:

quoted

On 02.09.25 09:59, Fuad Tabba wrote:

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Hi Patrick,

On Mon, 1 Sept 2025 at 15:56, Roy, Patrick [off-list ref] wrote:

quoted

On Mon, 2025-09-01 at 14:54 +0100, "Roy, Patrick" wrote:

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Hi Fuad!

On Thu, 2025-08-28 at 11:21 +0100, Fuad Tabba wrote:

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Hi Patrick,

On Thu, 28 Aug 2025 at 10:39, Roy, Patrick [off-list ref] wrote:

quoted

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/ (local).
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

quoted

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

--
Cheers

David / dhildenb