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Date: Thu, 5 Mar 2015 21:07:29 +0300
From: Pavel Emelyanov <xemul@parallels.com>
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Subject: Re: [Qemu-devel] [PATCH 14/21] userfaultfd:
 mcopy_atomic|mfill_zeropage: UFFDIO_COPY|UFFDIO_ZEROPAGE preparation
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To: Andrea Arcangeli <aarcange@redhat.com>, qemu-devel@nongnu.org, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-api@vger.kernel.org, Android Kernel Team <kernel-team@android.com>
Cc: Robert Love <rlove@google.com>, Dave Hansen <dave@sr71.net>, Jan Kara <jack@suse.cz>, Neil Brown <neilb@suse.de>, Stefan Hajnoczi <stefanha@gmail.com>, Andrew Jones <drjones@redhat.com>, Sanidhya Kashyap <sanidhya.gatech@gmail.com>, KOSAKI Motohiro <kosaki.motohiro@gmail.com>, Michel Lespinasse <walken@google.com>, Taras Glek <tglek@mozilla.com>, zhang.zhanghailiang@huawei.com, Juan Quintela <quintela@redhat.com>, Hugh Dickins <hughd@google.com>, Mel Gorman <mgorman@suse.de>, Sasha Levin <sasha.levin@oracle.com>, "Dr. David Alan Gilbert" <dgilbert@redhat.com>, "Huangpeng (Peter)" <peter.huangpeng@huawei.com>, Andres Lagar-Cavilla <andreslc@google.com>, Christopher Covington <cov@codeaurora.org>, Anthony Liguori <anthony@codemonkey.ws>, Paolo Bonzini <pbonzini@redhat.com>, "Kirill A. Shutemov" <kirill@shutemov.name>, Keith Packard <keithp@keithp.com>, Wenchao Xia <wenchaoqemu@gmail.com>, Andy Lutomirski <luto@amacapital.net>, Minchan Kim <minchan@kernel.org>, Dmitry Adamushko <dmitry.adamushko@gmail.com>, Johannes Weiner <hannes@cmpxchg.org>, Mike Hommey <mh@glandium.org>, Andrew Morton <akpm@linux-foundation.org>, Linus Torvalds <torvalds@linux-foundation.org>, Peter Feiner <pfeiner@google.com>

> +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