Re: [PATCH] mm: fix page_mkclean_one (was: 2.6.19 file content corruption on ext3)

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 <a.p.zijlstra@xxxxxxxxx>
---
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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