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Subject: Re: [Qemu-devel] [PATCH 05/21] userfaultfd: add vm_userfaultfd_ctx
	to the vm_area_struct
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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>

> 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);
> .
>