Re: [PATCH v3 01/15] mm/vma: introduce VMA anon page offset field and add helpers

On Wed, Jul 29, 2026 at 05:48:36PM +0100, Lorenzo Stoakes (ARM) wrote:
> This patch establishes fields within the vm_area_struct type to store the
> anonymous page offset of VMAs.
> 
> The anonymous page offset of a VMA is equal to vma->vm_start >> PAGE_SHIFT
> if they are unfaulted or were not remapped, otherwise it is equal to this
> value at the point of first fault.
> 
> Currently, anonymous folios belonging to CoW'd MAP_PRIVATE-mapped
> file-backed VMAs are tracked by their file offset. By adding anonymous
> offset as a property of VMAs, we can now track them by their anonymous page
> offset instead.
> 
> By tracking this, we provide the means by which to eliminate this
> inconsistency, and more importantly lay the foundations for future work for
> the scalable CoW anonymous rmap rework.
> 
> This patch simply adds the fields and some simple helpers. Subsequent
> patches will update mm code to make use of these fields correctly.
> 
> The fields chosen are packed in the VMA such that, for 64-bit kernel
> builds, no additional space is taken up.
> 
> The first field is present on cacheline 0 containing key VMA fields, and
> the second on cacheline 3, which contains file-backed reverse mapping
> fields.
> 
> Given the relative time spent accessing reverse mapping fields as well as
> updating them, there shouldn't be any performance impact here from false
> sharing.
> 
> Update the VMA userland tests to account for this change.
> 
> No callsites are updated yet, so no functional change intended.
> 
> Signed-off-by: Lorenzo Stoakes (ARM) <[email protected]>

Much clearer :], appreciate the rewording.

Reviewed-by: Gregory Price (Meta) <[email protected]>