Re: mmu_notifier: close hole in fork

[Andrea Arcangeli <andrea@qumranet.com>]

On Thu, Jan 31, 2008 at 02:01:43PM -0800, Christoph Lameter wrote:
> Talking to Robin and Jack we found taht we still have a hole during fork. 
> Fork may set a pte writeprotect. At that point the remote pte are 
> not marked readonly(!). Remote writes may occur to pages that are marked 
> readonly locally without this patch.

Good catch! This was missing also in my #v5 (KVM doesn't need that
because the only possible cows on sptes can be generated by ksm, but
it would have been a problem for GRU). The more I think about it, the
more I think GRU will run so nicely with follow_page taking zero
additional locking and with the nicely scalable PT lock that doesn't
require 1024 threads to trash on the same lock on 1024 different cpus
if they access addresses that aren't in the same naturally aligned 2M
chunk of virtual address space. I understood follow_page was
performance critical for GRU, if yes, then I hope my _dual_ approach
is by far the best for at least GRU (and KVM of course for the very
same reason), and of course it'll fit XPMEM too the moment you add
invalidate_range_start/end too.

Signed-off-by: Andrea Arcangeli <andrea@qumranet.com>

diff --git a/mm/memory.c b/mm/memory.c
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -494,6 +494,7 @@ static int copy_pte_range(struct mm_stru
 	spinlock_t *src_ptl, *dst_ptl;
 	int progress = 0;
 	int rss[2];
+	unsigned long start;
 
 again:
 	rss[1] = rss[0] = 0;
@@ -505,6 +506,7 @@ again:
 	spin_lock_nested(src_ptl, SINGLE_DEPTH_NESTING);
 	arch_enter_lazy_mmu_mode();
 
+	start = addr;
 	do {
 		/*
 		 * We are holding two locks at this point - either of them
@@ -525,6 +527,8 @@ again:
 	} while (dst_pte++, src_pte++, addr += PAGE_SIZE, addr != end);
 
 	arch_leave_lazy_mmu_mode();
+	if (is_cow_mapping(vma->vm_flags))
+		mmu_notifier(invalidate_pages, vma->vm_mm, start, addr);
 	spin_unlock(src_ptl);
 	pte_unmap_nested(src_pte - 1);
 	add_mm_rss(dst_mm, rss[0], rss[1]);

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