+ mm-huge_memory-conditionally-call-maybe_mkwrite-and-drop-pte_wrprotect-in-__split_huge_pmd_locked.patch added to mm-unstable branch

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

The patch titled
     Subject: mm/huge_memory: conditionally call maybe_mkwrite() and drop pte_wrprotect() in __split_huge_pmd_locked()
has been added to the -mm mm-unstable branch.  Its filename is
     mm-huge_memory-conditionally-call-maybe_mkwrite-and-drop-pte_wrprotect-in-__split_huge_pmd_locked.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-huge_memory-conditionally-call-maybe_mkwrite-and-drop-pte_wrprotect-in-__split_huge_pmd_locked.patch

This patch will later appear in the mm-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: David Hildenbrand <david@redhat.com>
Subject: mm/huge_memory: conditionally call maybe_mkwrite() and drop pte_wrprotect() in __split_huge_pmd_locked()
Date: Tue, 11 Apr 2023 16:25:12 +0200

No need to call maybe_mkwrite() to then wrprotect if the source PMD was not
writable.

It's worth nothing that this now allows for PTEs to be writable even if
the source PMD was not writable: if vma->vm_page_prot includes write
permissions.

As documented in commit 931298e103c2 ("mm/userfaultfd: rely on
vma->vm_page_prot in uffd_wp_range()"), any mechanism that intends to
have pages wrprotected (COW, writenotify, mprotect, uffd-wp, softdirty,
...) has to properly adjust vma->vm_page_prot upfront, to not include
write permissions. If vma->vm_page_prot includes write permissions, the
PTE/PMD can be writable as default.

This now mimics the handling in mm/migrate.c:remove_migration_pte() and in
mm/huge_memory.c:remove_migration_pmd(), which has been in place for a
long time (except that 96a9c287e25d ("mm/migrate: fix wrongly apply write
bit after mkdirty on sparc64") temporarily changed it).

Link: https://lkml.kernel.org/r/20230411142512.438404-7-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Cc: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: Hugh Dickins <hughd@google.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Yu Zhao <yuzhao@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/huge_memory.c |    5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

--- a/mm/huge_memory.c~mm-huge_memory-conditionally-call-maybe_mkwrite-and-drop-pte_wrprotect-in-__split_huge_pmd_locked
+++ a/mm/huge_memory.c
@@ -2233,11 +2233,10 @@ static void __split_huge_pmd_locked(stru
 				entry = pte_swp_mkuffd_wp(entry);
 		} else {
 			entry = mk_pte(page + i, READ_ONCE(vma->vm_page_prot));
-			entry = maybe_mkwrite(entry, vma);
+			if (write)
+				entry = maybe_mkwrite(entry, vma);
 			if (anon_exclusive)
 				SetPageAnonExclusive(page + i);
-			if (!write)
-				entry = pte_wrprotect(entry);
 			if (!young)
 				entry = pte_mkold(entry);
 			/* NOTE: this may set soft-dirty too on some archs */
_

Patches currently in -mm which might be from david@redhat.com are

mm-userfaultfd-fix-uffd-wp-handling-for-thp-migration-entries.patch
m68k-mm-use-correct-bit-number-in-_page_swp_exclusive-comment.patch
mm-userfaultfd-dont-consider-uffd-wp-bit-of-writable-migration-entries.patch
selftests-mm-reuse-read_pmd_pagesize-in-cow-selftest.patch
selftests-mm-mkdirty-test-behavior-of-ptepmd_mkdirty-on-vmas-without-write-permissions.patch
sparc-mm-dont-unconditionally-set-hw-writable-bit-when-setting-pte-dirty-on-64bit.patch
mm-migrate-revert-mm-migrate-fix-wrongly-apply-write-bit-after-mkdirty-on-sparc64.patch
mm-huge_memory-revert-partly-revert-mm-thp-carry-over-dirty-bit-when-thp-splits-on-pmd.patch
mm-huge_memory-conditionally-call-maybe_mkwrite-and-drop-pte_wrprotect-in-__split_huge_pmd_locked.patch