linux-kernel - Re: [PATCH v2 3/3] mm: Protect kernel pgtables in apply_to_pte

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Message-ID: <Z/fauW5hDSt+ciwr@li-008a6a4c-3549-11b2-a85c-c5cc2836eea2.ibm.com>
Date: Thu, 10 Apr 2025 16:50:33 +0200
From: Alexander Gordeev <agordeev@...ux.ibm.com>
To: Andrew Morton <akpm@...ux-foundation.org>,
        Andrey Ryabinin <ryabinin.a.a@...il.com>
Cc: Hugh Dickins <hughd@...gle.com>, Nicholas Piggin <npiggin@...il.com>,
        Guenter Roeck <linux@...ck-us.net>, Juergen Gross <jgross@...e.com>,
        Jeremy Fitzhardinge <jeremy@...p.org>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, kasan-dev@...glegroups.com,
        sparclinux@...r.kernel.org, xen-devel@...ts.xenproject.org,
        linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org,
        stable@...r.kernel.org
Subject: Re: [PATCH v2 3/3] mm: Protect kernel pgtables in
 apply_to_pte_range()

On Tue, Apr 08, 2025 at 06:07:32PM +0200, Alexander Gordeev wrote:

Hi Andrew,

> The lazy MMU mode can only be entered and left under the protection
> of the page table locks for all page tables which may be modified.

Heiko Carstens noticed that the above claim is not valid, since
v6.15-rc1 commit 691ee97e1a9d ("mm: fix lazy mmu docs and usage"),
which restates it to:

"In the general case, no lock is guaranteed to be held between entry and exit
of the lazy mode. So the implementation must assume preemption may be enabled"

That effectively invalidates this patch, so it needs to be dropped.

Patch 2 still could be fine, except -stable and Fixes tags and it does
not need to aim 6.15-rcX. Do you want me to repost it?

Thanks!