Re: update_mmu_cache() when write protecting pte.

[Hugh Dickins <hugh.dickins@tiscali.co.uk>]

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@redhat.com>
> 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@redhat.com>
> ---
>  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

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