+ mm-pagewalk-fix-bootstopping-regression-from-extra-pte_unmap.patch added to mm-hotfixes-unstable branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: mm/pagewalk: fix bootstopping regression from extra pte_unmap()
has been added to the -mm mm-hotfixes-unstable branch.  Its filename is
     mm-pagewalk-fix-bootstopping-regression-from-extra-pte_unmap.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-pagewalk-fix-bootstopping-regression-from-extra-pte_unmap.patch

This patch will later appear in the mm-hotfixes-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

Before you just go and hit "reply", please:
   a) Consider who else should be cc'ed
   b) Prefer to cc a suitable mailing list as well
   c) Ideally: find the original patch on the mailing list and do a
      reply-to-all to that, adding suitable additional cc's

*** Remember to use Documentation/process/submit-checklist.rst when testing your code ***

The -mm tree is included into linux-next via the mm-everything
branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
and is updated there every 2-3 working days

------------------------------------------------------
From: Hugh Dickins <hughd@google.com>
Subject: mm/pagewalk: fix bootstopping regression from extra pte_unmap()
Date: Sat, 2 Sep 2023 08:29:30 -0700 (PDT)

Mikhail reports early-6.6-based Fedora Rawhide not booting: "rcu_preempt
detected expedited stalls", minutes wait, and then hung_task splat while
kworker trying to synchronize_rcu_expedited().  Nothing logged to disk.

He bisected to my 6.6 a349d72fd9ef ("mm/pgtable: add rcu_read_lock() and
rcu_read_unlock()s"): but the one to blame is my 6.5 commit to fix the
espfix "bad pmd" warnings when booting x86_64 with CONFIG_EFI_PGT_DUMP=y.

Gaah, that added an "addr >= TASK_SIZE" check to avoid pte_offset_map(),
but failed to add the equivalent check when choosing to pte_unmap().

It's not a problem on 6.5 (for different reasons, it's harmless on both
64-bit and 32-bit), but becomes a bootstopper on 6.6 with the unbalanced
rcu_read_unlock() - RCU has a WARN_ON_ONCE for that, but it would have
scrolled off Mikhail's console too quickly.

Link: https://lkml.kernel.org/r/8321a347-4a38-526c-97fb-5999ceaf6dd@google.com
Fixes: 8b1cb4a2e819 ("mm/pagewalk: fix EFI_PGT_DUMP of espfix area")
Fixes: a349d72fd9ef ("mm/pgtable: add rcu_read_lock() and rcu_read_unlock()s")
Signed-off-by: Hugh Dickins <hughd@google.com>
Reported-by: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>
Closes: https://lore.kernel.org/linux-mm/CABXGCsNi8Tiv5zUPNXr6UJw6qV1VdaBEfGqEAMkkXE3QPvZuAQ@mail.gmail.com/
Tested-by: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>
Cc: Bagas Sanjaya <bagasdotme@gmail.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/pagewalk.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/mm/pagewalk.c~mm-pagewalk-fix-bootstopping-regression-from-extra-pte_unmap
+++ a/mm/pagewalk.c
@@ -58,7 +58,7 @@ static int walk_pte_range(pmd_t *pmd, un
 			pte = pte_offset_map(pmd, addr);
 		if (pte) {
 			err = walk_pte_range_inner(pte, addr, end, walk);
-			if (walk->mm != &init_mm)
+			if (walk->mm != &init_mm && addr < TASK_SIZE)
 				pte_unmap(pte);
 		}
 	} else {
_

Patches currently in -mm which might be from hughd@google.com are

mm-pagewalk-fix-bootstopping-regression-from-extra-pte_unmap.patch