Re: [PATCH 14/21] userfaultfd: mcopy_atomic|mfill_zeropage: UFFDIO_COPY|UFFDIO_ZEROPAGE preparation

Pavel Emelyanov
2015-03-05T18:07:29Z

> +static int mcopy_atomic_pte(struct mm_struct *dst_mm,
> + pmd_t *dst_pmd,
> + struct vm_area_struct *dst_vma,
> + unsigned long dst_addr,
> + unsigned long src_addr)
> +{
> + struct mem_cgroup *memcg;
> + pte_t _dst_pte, *dst_pte;
> + spinlock_t *ptl;
> + struct page *page;
> + void *page_kaddr;
> + int ret;
> +
> + ret = -ENOMEM;
> + page = alloc_page_vma(GFP_HIGHUSER_MOVABLE, dst_vma, dst_addr);
> + if (!page)
> + goto out;

Not a fatal thing, but still quite inconvenient. If there are two tasks that
have anonymous private VMAs that are still not COW-ed from each other, then
it will be impossible to keep the pages shared with userfault. Thus if we do
post-copy memory migration for tasks, then these guys will have their
memory COW-ed.

Thanks,
Pavel