From mboxrd@z Thu Jan  1 00:00:00 1970
From: Pavel Emelyanov <xemul@parallels.com>
Subject: Re: [PATCH 14/21] userfaultfd: mcopy_atomic|mfill_zeropage: UFFDIO_COPY|UFFDIO_ZEROPAGE
 preparation
Date: Thu, 5 Mar 2015 21:07:29 +0300
Message-ID: <54F89B61.308@parallels.com>
References: <1425575884-2574-1-git-send-email-aarcange@redhat.com> <1425575884-2574-15-git-send-email-aarcange@redhat.com>
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Cc: "Kirill A. Shutemov" <kirill@shutemov.name>, Sanidhya Kashyap
	<sanidhya.gatech@gmail.com>, <zhang.zhanghailiang@huawei.com>, Linus Torvalds
	<torvalds@linux-foundation.org>, Andres Lagar-Cavilla <andreslc@google.com>,
	Dave Hansen <dave@sr71.net>, Paolo Bonzini <pbonzini@redhat.com>, Rik van
 Riel <riel@redhat.com>, Mel Gorman <mgorman@suse.de>, Andy Lutomirski
	<luto@amacapital.net>, Andrew Morton <akpm@linux-foundation.org>, Sasha Levin
	<sasha.levin@oracle.com>, Hugh Dickins <hughd@google.com>, Peter Feiner
	<pfeiner@google.com>, "Dr. David Alan Gilbert" <dgilbert@redhat.com>,
	Christopher Covington <cov@codeaurora.org>, Johannes Weiner
	<hannes@cmpxchg.org>, Robert Love <rlove@google.com>, Dmitry Adamushko
	<dmitry.adamushko@gmail.com>, Neil Brown <neilb@suse.de>, Mike Hommey
	<mh@glandium.org>, Taras Glek <tglek@mozilla.com>, Jan Kara <jack@suse.cz>,
	KOSAKI Motohiro <kosaki.mo
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>
Return-path: <owner-linux-mm@kvack.org>
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List-Id: kvm.vger.kernel.org

> +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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