mm, thp: Do not make pmd/pud dirty without a reason

Currently we make page table entries dirty all the time regardless of
access type and don't even consider if the mapping is write-protected.
The reasoning is that we don't really need dirty tracking on THP and
making the entry dirty upfront may save some time on first write to the
page.

Unfortunately, such approach may result in false-positive
can_follow_write_pmd() for huge zero page or read-only shmem file.

Let's only make page dirty only if we about to write to the page anyway
(as we do for small pages).

I've restructured the code to make entry dirty inside
maybe_p[mu]d_mkwrite(). It also takes into account if the vma is
write-protected.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 1 insertions, 1 deletions

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index ea4ff259b671..db43dc8a8ae6 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1057,7 +1057,7 @@ static void collapse_huge_page(struct mm_struct *mm,
 	pgtable = pmd_pgtable(_pmd);
 
 	_pmd = mk_huge_pmd(new_page, vma->vm_page_prot);
-	_pmd = maybe_pmd_mkwrite(pmd_mkdirty(_pmd), vma);
+	_pmd = maybe_pmd_mkwrite(_pmd, vma, false);
 
 	/*
 	 * spin_lock() below is not the equivalent of smp_wmb(), so