Messages in this thread Patch in this message | | | Subject | Re: [PATCH] mm: fix page_mkclean_one (was: 2.6.19 file content corruption on ext3) | From | Peter Zijlstra <> | Date | Wed, 20 Dec 2006 14:56:18 +0100 |
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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@chello.nl> --- 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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