On Jul 26, 2007, at 11:33 PM, Peter Zijlstra wrote:
On Thu, 2007-07-26 at 17:23 -0700, Andrew Morton wrote:
On Thu, 26 Jul 2007 16:52:44 -0700 Suleiman Souhlal
<[email protected]> wrote:
make_pages_present() is dirtying mlocked pages if the VMA is
writable, even
though it shouldn't, by telling get_user_pages() to simulate a
write fault.
A simple way to test this is to mlock a multi-GB file, and then
sync.
The sync will take a long time.
ugh, how bad of us.
As far as I can see, it should be safe to just not simulate a
write fault.
We pass in "write=1" to force a COW. This is because we want to
do all
that memory allocation at mlock()-time, not later on, when the app
writes
to the page.
<snip patch>
So something sterner will need to be done. I guess the
write_access arg to
handle_mm_fault() would need to become a three-value thing.
That would be most painfull. Can't we simply set write=0 for shared
mappings? Those won't have COW to break and are the onces that do
requires writeback. Anonymous and private mappings do COW but will
never
writeback and are thus save to touch with write=1.
How about something like this:
Signed-off-by: Peter Zijlstra <[email protected]>
---
mm/memory.c | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)
Index: linux-2.6/mm/memory.c
===================================================================
--- linux-2.6.orig/mm/memory.c
+++ linux-2.6/mm/memory.c
@@ -2716,7 +2716,12 @@ int make_pages_present(unsigned long add
vma = find_vma(current->mm, addr);
if (!vma)
return -1;
- write = (vma->vm_flags & VM_WRITE) != 0;
+ /*
+ * We want to touch writable mappings with a write fault in otder
+ * to break COW, except for shared mappings because these don't COW
+ * and we would not want to dirty them for nothing.
+ */
+ write = (vma->vm_flags & VM_WRITE|VM_SHARED) == VM_WRITE;
BUG_ON(addr >= end);
BUG_ON(end > vma->vm_end);
len = DIV_ROUND_UP(end, PAGE_SIZE) - addr/PAGE_SIZE;
Oops, I completely forgot about this. Sorry about that.
It works for me (as long as you put parens around VM_WRITE|VM_SHARED).
Please consider committing it. :-)
Signed-off-by: Suleiman Souhlal <[email protected]>
-- Suleiman
-
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