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Date: Mon, 19 Oct 2015 16:30:22 -0700
From: Davidlohr Bueso <dave@stgolabs.net>
Subject: Re: [PATCH 2/12] mm: rmap use pte lock not mmap_sem to set
 PageMlocked
Message-ID: <20151019233022.GA27292@linux-uzut.site>
References: <alpine.LSU.2.11.1510182132470.2481@eggly.anvils>
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 <56248C5B.3040505@suse.cz>
 <alpine.LSU.2.11.1510190341490.3809@eggly.anvils>
 <20151019131308.GB15819@node.shutemov.name>
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To: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Hugh Dickins <hughd@google.com>, Vlastimil Babka <vbabka@suse.cz>, Andrew Morton <akpm@linux-foundation.org>, Christoph Lameter <cl@linux.com>, "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>, Rik van Riel <riel@redhat.com>, Oleg Nesterov <oleg@redhat.com>, Sasha Levin <sasha.levin@oracle.com>, Andrey Konovalov <andreyknvl@google.com>, Dmitry Vyukov <dvyukov@google.com>, KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>, linux-mm@kvack.org

On Mon, 19 Oct 2015, Kirill A. Shutemov wrote:

>I think we need to have at lease WRITE_ONCE() everywhere we update
>vm_flags and READ_ONCE() where we read it without mmap_sem taken.

Given the CPU-CPU interaction, lockless/racy vm_flag checks should
actually be using barrier pairing, afaict. This is expensive obviously,
but I cannot recall what other places we do lockless games with vm_flags.

Perhaps its time to introduce formal helpers around vma->vm_flags such
that we can encapsulate such things: __vma_[set/read]_vmflags() or whatever
that would be used for those racy scenarios.

Thanks,
Davidlohr

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