On Wed, 2006-12-20 at 13:00 +0000, Hugh Dickins wrote:
> On Wed, 20 Dec 2006, Peter Zijlstra wrote:
> >
> > fix page_mkclean_one()
>
> Congratulations on getting to the bottom of it, Peter (if you have:
> I haven't digested enough of the thread to tell).
Well, I thought I understood, you just shattered that.
> I'm mostly offline at
> present, no time for dialogue, I'll throw out a few remarks and run...
I wondered where you were ;-) Enjoy your time away from the computer.
> >
> > it had several issues:
> > - it failed to flush the cache
>
> It's unclear to me why it should need to flush the cache, but I don't
> know much about that, and mprotect does flush the cache in advance -
> I think others will tell you that if it does need to be flushed,
I was still thinking about why exactly, but indeed since mprotect does I
thought it prudent to also do it.
> it must
> be flushed while there's still a valid pte (on some arches at least).
Ah, good point, makes sense I guess.
> > - it failed to flush the tlb
>
> Eh? It flushed the TLB inside ptep_establish, didn't it?
> I guess you mean you've found a race before it flushed the TLB.
Hmm, quite right indeed. I missed that. So moving the flush inside the
pte cleared section closed a race. It seems I must have a long hard look
at these architecture manuals...
> > - it failed to do s390 (s390 guys, please verify this is now correct)
>
> Hmm, I thought we cleared it with them back at the time.
/me queries mail folder...
can't seem to find it.
> >
> > Also, clear in a loop to ensure SMP safeness as suggested by Arjan.
>
> Yikes. Well, please compare with mprotect's change_pte_range. I think
> I took that as the relevant standard when checking your implementation,
> and back then satisfied myself that what you were doing was equivalent.
> If page_mkclean_one is now agreed to be significantly defective, then
> I suspect change_pte_range is also; perhaps others too.
Arjan argued that mprotect and msync would mostly race with themselves
in userspace.
> (But I haven't found time to do more than skim through the thread,
> I've not thought through the issues at all: I am surprised that it's
> now found defective, we looked at it long and hard back then.)
---
page_mkclean_one() fix
it had several issues:
- it failed to flush the cache
- a race wrt tlb flushing
- it failed to do s390 (s390 guys, please verify this is now correct)
Also, clear in a loop to ensure SMP safeness as suggested by Arjan.
Signed-off-by: Peter Zijlstra <[email protected]>
---
mm/rmap.c | 23 +++++++++++++----------
1 file changed, 13 insertions(+), 10 deletions(-)
Index: linux-2.6/mm/rmap.c
===================================================================
--- linux-2.6.orig/mm/rmap.c
+++ linux-2.6/mm/rmap.c
@@ -432,7 +432,7 @@ static int page_mkclean_one(struct page
{
struct mm_struct *mm = vma->vm_mm;
unsigned long address;
- pte_t *pte, entry;
+ pte_t *pte;
spinlock_t *ptl;
int ret = 0;
@@ -444,17 +444,20 @@ static int page_mkclean_one(struct page
if (!pte)
goto out;
- if (!pte_dirty(*pte) && !pte_write(*pte))
- goto unlock;
+ while (pte_dirty(*pte) || pte_write(*pte)) {
+ pte_t entry;
- entry = ptep_get_and_clear(mm, address, pte);
- entry = pte_mkclean(entry);
- entry = pte_wrprotect(entry);
- ptep_establish(vma, address, pte, entry);
- lazy_mmu_prot_update(entry);
- ret = 1;
+ flush_cache_page(vma, address, pte_pfn(*pte));
+ entry = ptep_get_and_clear(mm, address, pte);
+ flush_tlb_page(vma, address);
+ (void)page_test_and_clear_dirty(page); /* do the s390 thing */
+ entry = pte_wrprotect(entry);
+ entry = pte_mkclean(entry);
+ set_pte_at(vma, address, pte, entry);
+ lazy_mmu_prot_update(entry);
+ ret = 1;
+ }
-unlock:
pte_unmap_unlock(pte, ptl);
out:
return ret;
-
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