On Mon, Aug 03, 2026 at 12:52:42PM +0200, David Hildenbrand (Arm) wrote: > 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() ?
No this would be incorrect. A read-only mapping would become unmergeable here. So this is something apart from the rmap aspect, and it is a contract that upon move of an unfaulted mapping (which for read-only anon would always be unfaulted) that vma->vm_pgoff is updated. This is later in this series extended to MAP_PRIVATE file-backed mappings also. Also it would break the faulted_in_anon_vma logic and result in incorrect VM_WARN_ON_ONCE() invocations below. As for other -> is_cow_mapping() changes: * linear_anon_page_index() - [as discussed already] - fine * vma_anon_address() - convert to is_cow_mapping() [have to be faulted in to hit it] * needs_adjacent_anon_pgoff() - don't convert - same merging argument as above. > > > > + 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]> Thanks! > > -- > Cheers, > > David -- Cheers, Lorenzo
