On Mon, Aug 03, 2026 at 11:28:28AM +0200, David Hildenbrand (Arm) wrote: > On 7/29/26 18:48, Lorenzo Stoakes (ARM) wrote: > > This patch establishes fields within the vm_area_struct type to store the > > anonymous page offset of VMAs. > > Nit: Avoid "This patch". Just say "Establish ..." (I was surprised to find > that even Documentation/process/submitting-patches.rst document this)
Yeah ever since Boris nagged me I've (possibly reluctantly? :P) found that using the active voice is a lot clearer and have been using that more so. Will fix up on respin. > > > > > 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]> > > --- > > [...] > > > +static inline void __vma_set_anon_pgoff(struct vm_area_struct *vma, > > pgoff_t pgoff) > > +{ > > +#ifdef CONFIG_64BIT > > + vma->__vm_anon_pgoff_hi = pgoff >> 32; > > +#endif > > + vma->__vm_anon_pgoff_lo = pgoff & GENMASK(31, 0); > > +} > > + > > +static inline void vma_set_anon_pgoff(struct vm_area_struct *vma, pgoff_t > > pgoff) > > +{ > > + vma_assert_can_modify(vma); > > + __vma_set_anon_pgoff(vma, pgoff); > > +} > > + > > static inline void vma_add_pgoff(struct vm_area_struct *vma, pgoff_t delta) > > { > > vma_assert_can_modify(vma); > > diff --git a/mm/vma_init.c b/mm/vma_init.c > > index 715feee283f0..baa7e82f47e3 100644 > > --- a/mm/vma_init.c > > +++ b/mm/vma_init.c > > @@ -51,6 +51,7 @@ static void vm_area_init_from(const struct vm_area_struct > > *src, > > dest->vm_end = src->vm_end; > > dest->anon_vma = src->anon_vma; > > dest->vm_pgoff = vma_start_pgoff(src); > > + __vma_set_anon_pgoff(dest, vma_start_anon_pgoff(src)); > > It would be cleaner if both interfaces would either contain "start" or not > contain it. > > The former would be better > > __vma_set_start_anon_pgoff(dest, vma_start_anon_pgoff(src)); Ack that's fair, will update it. > > Acked-by: David Hildenbrand (Arm) <[email protected]> Thanks! > > -- > Cheers, > > David -- Cheers, Lorenzo
