linux-kernel - Re: [PATCH 14/21] userfaultfd: mcopy_atomic|mfill_zeropage: UFFDIO_COPY|UFFDIO

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Date:	Thu, 5 Mar 2015 21:07:29 +0300
From:	Pavel Emelyanov <xemul@...allels.com>
To:	Andrea Arcangeli <aarcange@...hat.com>, <qemu-devel@...gnu.org>,
	<kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<linux-mm@...ck.org>, <linux-api@...r.kernel.org>,
	Android Kernel Team <kernel-team@...roid.com>
CC:	"Kirill A. Shutemov" <kirill@...temov.name>,
	Sanidhya Kashyap <sanidhya.gatech@...il.com>,
	<zhang.zhanghailiang@...wei.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andres Lagar-Cavilla <andreslc@...gle.com>,
	Dave Hansen <dave@...1.net>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mgorman@...e.de>,
	Andy Lutomirski <luto@...capital.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Sasha Levin <sasha.levin@...cle.com>,
	Hugh Dickins <hughd@...gle.com>,
	Peter Feiner <pfeiner@...gle.com>,
	"Dr. David Alan Gilbert" <dgilbert@...hat.com>,
	Christopher Covington <cov@...eaurora.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Robert Love <rlove@...gle.com>,
	Dmitry Adamushko <dmitry.adamushko@...il.com>,
	Neil Brown <neilb@...e.de>, Mike Hommey <mh@...ndium.org>,
	Taras Glek <tglek@...illa.com>, Jan Kara <jack@...e.cz>,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	Michel Lespinasse <walken@...gle.com>,
	Minchan Kim <minchan@...nel.org>,
	Keith Packard <keithp@...thp.com>,
	"Huangpeng (Peter)" <peter.huangpeng@...wei.com>,
	Anthony Liguori <anthony@...emonkey.ws>,
	Stefan Hajnoczi <stefanha@...il.com>,
	Wenchao Xia <wenchaoqemu@...il.com>,
	Andrew Jones <drjones@...hat.com>,
	Juan Quintela <quintela@...hat.com>
Subject: Re: [PATCH 14/21] userfaultfd: mcopy_atomic|mfill_zeropage: UFFDIO_COPY|UFFDIO_ZEROPAGE
 preparation

> +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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