linux-kernel - Re: pipe/page fault oddness.

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Message-ID: <CA+55aFx691Ymqnw1oBUGJdsKsSrKvJ9sVVjYRwKd8QEhTpos3A@mail.gmail.com>
Date:	Thu, 2 Oct 2014 09:01:38 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	"Kirill A. Shutemov" <kirill@...temov.name>
Cc:	Sasha Levin <sasha.levin@...cle.com>,
	Hugh Dickins <hughd@...gle.com>, Dave Jones <davej@...hat.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Rik van Riel <riel@...hat.com>,
	Ingo Molnar <mingo@...hat.com>,
	Michel Lespinasse <walken@...gle.com>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Mel Gorman <mgorman@...e.de>
Subject: Re: pipe/page fault oddness.

On Thu, Oct 2, 2014 at 7:25 AM, Kirill A. Shutemov <kirill@...temov.name> wrote:
>
> I don't see what prevents the code to make zero page writable here.
> We need at least pmd = pmd_wrprotect(pmd) before set_pmd_at();

Do we? If it's the zero page, it had better be an anonymous mapping,
and vm_page_prot had better not be writable.

Anonymous pages don't _start_ out writable, we explicitly make them so
with code like

        if (vma->vm_flags & VM_WRITE)
                entry = pte_mkwrite(pte_mkdirty(entry));

so it should be fine to just use "pmd_modify(pmd, vma->vm_page_prot);" directly.

But hey, this is the kind of thing that maybe I'm missing something on..

              Linus
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