diff options
authorLinus Torvalds <torvalds@linux-foundation.org>2018-07-09 13:19:49 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2018-08-25 13:15:03 -0700
commit1b2de5d039c883c9d44ae5b2b6eca4ff9bd82dac (patch)
parente0fcfe1f1acf2f129e0809ea89b9187e89e84246 (diff)
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 c3ad2c3b02e9 ("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>
1 files changed, 1 insertions, 1 deletions
diff --git a/mm/memory.c b/mm/memory.c
index 83aef222f11b..c467102a5cbc 100644
--- 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);