mm/cow: don't bother write protecting already write-protected pages

This is not normally noticeable, but repeated forks are unnecessarily
expensive because they repeatedly dirty the parent page tables during
the page table copy operation.

It's trivial to just avoid write protecting the page table entry if it
was already not writable.

This patch was inspired by

    https://bugzilla.kernel.org/show_bug.cgi?id=200447

which points to an ancient "waste time re-doing fork" issue in the
presence of lots of signals.

That bug was fixed by Eric Biederman's signal handling series
culminating in commit bc3a7d03c31e ("signal: Don't restart fork when
signals come in"), but the unnecessary work for repeated forks is still
work just fixing, particularly since the fix is trivial.

Cc: Eric Biederman <ebiederm@xmission.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/memory.c b/mm/memory.c
index 83aef222f11b3f1d847279e13f43358566a22b43..c467102a5cbc5de5ab2ea4dff740f2611c1124a4 100644 (file)
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1022,7 +1022,7 @@ copy_one_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm,
         * If it's a COW mapping, write protect it both
         * in the parent and the child
         */
-       if (is_cow_mapping(vm_flags)) {
+       if (is_cow_mapping(vm_flags) && pte_write(pte)) {
                ptep_set_wrprotect(src_mm, addr, src_pte);
                pte = pte_wrprotect(pte);
        }