Added linux-arch to Cc list. On Wed, 23 Sep 2009, Izik Eidus wrote: > Hi, Hugh just found out that ksm was not calling to update_mmu_cache() > after it set new pte when it changed ptes mapping to point into the new > shared-readonly page (ksmpage). > > It is understandable that it is a bug and ksm have to call it right > after set_pte_at_notify() get called, but the question is: does ksm > have to call it only there or should it call it even when it > write-protect pte (while not changing the physical address the pte is > pointing to). I'm currently inclining to the view that it's only necessary to call update_mmu_cache() in faulting paths (as cachetlb.txt says), and would just be a waste of time and cache to call it from KSM (which, like mprotect, has no reason to suppose that the pte will soon be faulted). Documentation/cachetlb.txt is specific when it says: At the end of every page fault, this routine is invoked... But less so when it says: A port may use this information in any way it so chooses. In private mail, I was worrying about how mprotect does not call update_mmu_cache, and thinking of the race when mprotect makes a pte writable while a write access is coming down through handle_pte_fault: such that handle_pte_fault skips its update_mmu_cache: but hadn't noticed the "else" there, which will flush_tlb_page to reset the condition, so we don't have repeated faults on those architectures which are liable to that if the update_mmu_cache() is missed. I think now that neither replace_page() nor write_protect_page() should update_mmu_cache(); but my mind may change in a few moments time ;) Hugh > > I am asking this question because it seems that fork() dont call it... > > (below a patch that fix the problem in case we need it just when we > change the physical mapping, if we need it even when we write protect > the pages, then we need to add another update_mmu_cache() call) > > Thanks. > > From 82d27f67a8b20767dc6119422189f73b52168c8d Mon Sep 17 00:00:00 2001 > From: Izik Eidus <ieidus@xxxxxxxxxx> > Date: Wed, 23 Sep 2009 22:37:34 +0300 > Subject: [PATCH] ksm: add update_mmu_cache() when changing pte mapping. > > This patch add update_mmu_cache() call right after set_pte_at_notify() > Without this function ksm is probably broken for powerpc and sparc archs. > > (Noticed by Hugh Dickins) > > Signed-off-by: Izik Eidus <ieidus@xxxxxxxxxx> > --- > mm/ksm.c | 1 + > 1 files changed, 1 insertions(+), 0 deletions(-) > > diff --git a/mm/ksm.c b/mm/ksm.c > index f7edac3..e8d16eb 100644 > --- a/mm/ksm.c > +++ b/mm/ksm.c > @@ -719,6 +719,7 @@ static int replace_page(struct vm_area_struct *vma, struct page *oldpage, > flush_cache_page(vma, addr, pte_pfn(*ptep)); > ptep_clear_flush(vma, addr, ptep); > set_pte_at_notify(mm, addr, ptep, mk_pte(newpage, prot)); > + update_mmu_cache(vma, addr, pte); > > page_remove_rmap(oldpage); > put_page(oldpage); > -- > 1.5.6.5 -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
