linux-kernel - Re: [PATCH 05/21] userfaultfd: add vm_userfaultfd_ctx to the vm_area

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Date:	Thu, 5 Mar 2015 20:48:22 +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 05/21] userfaultfd: add vm_userfaultfd_ctx to the vm_area_struct

> diff --git a/kernel/fork.c b/kernel/fork.c
> index cf65139..cb215c0 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -425,6 +425,7 @@ static int dup_mmap(struct mm_struct *mm, struct mm_struct *oldmm)
>  			goto fail_nomem_anon_vma_fork;
>  		tmp->vm_flags &= ~VM_LOCKED;
>  		tmp->vm_next = tmp->vm_prev = NULL;
> +		tmp->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;

This creates an interesting effect when the userfaultfd is used outside of
the process which created and activated one. If I try to monitor the memory
usage of one task with another, once the first task fork()-s, its child
begins to see zero-pages in the places where the monitor task was supposed
to insert pages with data.

>  		file = tmp->vm_file;
>  		if (file) {
>  			struct inode *inode = file_inode(file);
> .
> 

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