On Thu, 2009-09-24 at 13:39 +0100, Hugh Dickins wrote: > 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). I tend to agree. The way we use update_mmu_cache() on ppc for example is to pre-fault in the hash table or the TLB. Typically this is used to avoid a second fault (TLB miss or hash miss) after a page fault. I think it would be detrimental to have it called more often in cases that aren't very likely to be accessed right away. > 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 ;) Heh. That's VM for you :-) Cheers, Ben. > 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, send a message with 'unsubscribe linux-mm' in > the body to majordomo@xxxxxxxxxx For more info on Linux MM, > see: http://www.linux-mm.org/ . > Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a> -- 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
