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

[Pavel Emelyanov]

> +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

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