Re: [PATCH v4 03/20] mm: introduce linear_anon_page_index()

On Sat, Aug 08, 2026 at 05:49:48PM -0700, Suren Baghdasaryan wrote:

On Thu, Aug 6, 2026 at 1:23 PM Lorenzo Stoakes (ARM) [off-list ref] wrote:

quoted

This function provides the anonymous equivalent of linear_page_index(),
instead offsetting based on the anonymous page offset of the VMA.

It is valid only for anonymous or MAP_PRIVATE file-backed mappings, in
other words CoW mappings.

For pure anon VMAs, this will be equal to linear_page_index().

Assert that both of these invariants are true In linear_anon_page_index()

s/In/in

Ack. I ran these patches through claude several times and it didn't get this ;)
the machines are not taking over just yet...

Andrew - could you change this in-place? Thanks!

quoted

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index c6fc783aaee5..0adfa6605653 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h

@@ -1094,10 +1094,44 @@ static inline pgoff_t linear_page_delta(const struct vm_area_struct *vma,
 static inline pgoff_t linear_page_index(const struct vm_area_struct *vma,
                                        const unsigned long address)
 {
-       pgoff_t pgoff;
+       return linear_page_delta(vma, address) + vma_start_pgoff(vma);
+}
+
+static inline pgoff_t __linear_anon_page_index(const struct vm_area_struct *vma,
+               const unsigned long address)
+{
+       return linear_page_delta(vma, address) + vma_start_anon_pgoff(vma);
+}
+
+/**
+ * linear_anon_page_index() - Determine the absolute anonymous page offset of
+ * @address within @vma.
+ * @vma: An anonymous or MAP_PRIVATE file-backed VMA in which @address resides.
+ * @address: The address whose absolute page offset is required.
+ *
+ * This returns the anonymous page offset of @address, which is the page offset
+ * the address possessed at the time the VMA was first faulted.
+ *
+ * For anonymous mappings, this returns the same value as linear_page_index().
+ *
+ * For MAP_PRIVATE file-backed mappings, this returns the anonymous page offset
+ * of @address, which is the page offset the address possessed at the time the
+ * VMA was first faulted.
+ *
+ * It is not valid to call this function for shared file-backed mappings.
+ *
+ * Returns: The absolute anonymous page offset of @address within @vma.
+ */
+static inline pgoff_t linear_anon_page_index(const struct vm_area_struct *vma,
+               const unsigned long address)
+{
+       const pgoff_t pgoff = __linear_anon_page_index(vma, address);
+
+       VM_WARN_ON_ONCE(!vma_is_cow_mapping(vma));

As discussed off the list, this assertion works even for read-only
anon mappings because, as you said, "there's no such thing as a pure
anon mapping that is !VMA_MAYWRITE_BIT, there is no way to achieve
that."
It took me some time to see that after reading your discussion with
David at [1] and [2], and I don't think it's very intuitive. Could you
please add a comment explaining that? Maybe add a comment at
vma_is_cow_mapping() definition since you are using the same assertion
in another patch of this series?

Sure will do! It is very confusing stuff (and I was duly confused also at
first).

To save a respin (unless there's more stuff I need to address) I will probably
reply to the relevant patch with a proposed new comment.

[1] https://lore.kernel.org/linux-mm/anBjpdMicqaGADr-@lucifer/ (local)
[2] https://lore.kernel.org/linux-mm/anLyhaI6kW-EWk13@lucifer/ (local)

--
Cheers, Lorenzo