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

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Message-ID: <e679d3fe-be8e-d7c0-798a-df32587553ed@redhat.com>
Date:   Wed, 14 Dec 2022 11:59:35 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Peter Xu <peterx@...hat.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Cc:     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 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.

> 
> 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().

> 
> Signed-off-by: Peter Xu <peterx@...hat.com>
> ---
> 
> Note: this patch should be able to apply cleanly with/without the other
> mm/migrate patch, or David's vm_page_prot changes.
> ---
>   arch/x86/include/asm/pgtable.h | 24 ++++++++++++------------
>   include/asm-generic/hugetlb.h  | 16 ++++++++--------
>   mm/huge_memory.c               |  8 +++-----
>   mm/hugetlb.c                   |  4 ++--
>   mm/memory.c                    |  8 +++-----
>   mm/mprotect.c                  |  6 ++----
>   mm/userfaultfd.c               | 18 ++----------------
>   7 files changed, 32 insertions(+), 52 deletions(-)

It's certainly a cleanup, even though we might unnecessarily wrprotect 
(I don't think we care).

-- 
Thanks,

David / dhildenb