linux-kernel - Re: [PATCH] mm/uffd: Always wr-protect pte in pte|pmd_mkuffd

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Date:   Wed, 14 Dec 2022 09:26:42 -0500
From:   Peter Xu <peterx@...hat.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrea Arcangeli <aarcange@...hat.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Hugh Dickins <hughd@...gle.com>,
        Ives van Hoorne <ives@...esandbox.io>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] mm/uffd: Always wr-protect pte in pte|pmd_mkuffd_wp()

On Wed, Dec 14, 2022 at 11:59:35AM +0100, David Hildenbrand wrote:
> On 08.12.22 20:46, Peter Xu wrote:
> > This patch is a cleanup to always wr-protect pte/pmd in mkuffd_wp paths.
> > 
> > The reasons I still think this patch is worthwhile, are:
> > 
> >    (1) It is a cleanup already; diffstat tells.
> > 
> >    (2) It just feels natural after I thought about this, if the pte is uffd
> >        protected, let's remove the write bit no matter what it was.
> > 
> >    (2) Since x86 is the only arch that supports uffd-wp, it also redefines
> >        pte|pmd_mkuffd_wp() in that it should always contain removals of
> >        write bits.  It means any future arch that want to implement uffd-wp
> >        should naturally follow this rule too.  It's good to make it a
> >        default, even if with vm_page_prot changes on VM_UFFD_WP.
> > 
> >    (3) It covers more than vm_page_prot.  So no chance of any potential
> >        future "accident" (like pte_mkdirty() sparc64 or loongarch, even
> >        though it just got its pte_mkdirty fixed <1 month ago).  It'll be
> >        fairly clear when reading the code too that we don't worry anything
> >        before a pte_mkuffd_wp() on uncertainty of the write bit.
> 
> Don't necessarily agree with (3). If you'd have a broken pte_mkdirty() and
> do the pte_mkdirty() after pte_mkuffd_wp() it would still be broken. Because
> sparc64 and loongarch are simply broken.

That's why I mentioned on the order of operations matters.

> 
> > 
> > We may call pte_wrprotect() one more time in some paths (e.g. thp split),
> > but that should be fully local bitop instruction so the overhead should be
> > negligible.
> > 
> > Although this patch should logically also fix all the known issues on
> > uffd-wp too recently on either page migration or numa balancing, but this
> > is not the plan for that fix.  So no fixes, and stable doesn't need this.
> 
> I don't see how this would fix do_numa_page(), where we only do a
> pte_modify().

Yes, this patch won't, because it's a pure cleanup.  Otherwise we need
another line of wr-protect in numa recover path.

I can remove that sentence in v2 commit log.

-- 
Peter Xu