Commit 1b2de5d0 authored Jul 09, 2018 by Linus Torvalds

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 c3ad2c3b ("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>

parent e0fcfe1f

mm/memory.c

+1 −1

Original line number	Diff line number	Diff line
		@@ -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);
		}

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