Re: [PATCH v3 06/15] mm: propagate VMA anonymous page offset on map, remap, split + merge

On 7/29/26 18:48, Lorenzo Stoakes (ARM) wrote:
> We must correctly update VMA anonymous page offset state on all VMA
> operations that would result in it changing, with special attention given
> to remapping.
> 
> We cover most cases by simply updating vma_set_range() to do so (with a new
> anonymous page offset parameter), but also notably must update the merging
> and mapping logic to propagate this parameter correctly.
> 
> The remap logic remains the same - we may update the anonymous page offset
> if the VMA is unfaulted, but now this applies to MAP_PRIVATE file-backed
> mappings too, so we update the code to reflect this.
> 
> Note that we use __linear_anon_page_index() upon remap as the VMA may be
> shared, in order that we update the field consistently regardless of VMA
> type.
> 
> Similarly, pass through anon page offset to the merge logic, updating the
> vma_merge_struct struct to propagate it, and also use
> __linear_anon_page_index() to obtain the anonymous page index so it can be
> safely used for both shared and MAP_PRIVATE file-backed mappings.
> 
> Finally, we update insert_vm_struct() to correctly set the anonymous page
> offset on insertion of a VMA.
> 
> We simply ensure state is correctly propagated here, so no functional
> changes are intended.
> 
> Also while we're here, replace a VM_BUG_ON_VMA() with a
> VM_WARN_ON_ONCE_VMA().
> 
> Also update VMA userland tests to reflect this change.
> 
> Signed-off-by: Lorenzo Stoakes (ARM) <[email protected]>


[...]

>  	struct vm_area_struct *vma = *vmap;
>  	unsigned long vma_start = vma->vm_start;
> @@ -1919,11 +1929,14 @@ struct vm_area_struct *copy_vma(struct vm_area_struct **vmap,
>  	VMG_VMA_STATE(vmg, &vmi, NULL, vma, addr, addr + len);
>  
>  	/*
> -	 * If anonymous vma has not yet been faulted, update new pgoff
> -	 * to match new location, to increase its chance of merging.
> +	 * If a vma has not yet been faulted, update its anonymous pgoff to
> +	 * match the new location to increase its chance of merging.
>  	 */
> -	if (unlikely(vma_is_anonymous(vma) && !vma->anon_vma)) {
> -		pgoff = addr >> PAGE_SHIFT;
> +	if (!vma->anon_vma && !vma_test(vma, VMA_SHARED_BIT)) {

Could we also use is_cow_mapping() ?


> +		anon_pgoff = addr >> PAGE_SHIFT;
> +
> +		if (vma_is_anonymous(vma))
> +			pgoff = anon_pgoff;
>  		faulted_in_anon_vma = false;
>  	}
>  

Acked-by: David Hildenbrand (Arm) <[email protected]>

-- 
Cheers,

David