Re: [RFC PATCH 01/11] nios2: update_mmu_cache clear the old entry from the TLB

[Nicholas Piggin <npiggin@gmail.com>]

Hi,

Did you get a chance to look at these?

This first patch 1/11 solves the lockup problem that Guenter reported
with my changes to core mm code. So I plan to resubmit my patches
to Andrew's -mm tree with this patch to avoid nios2 breakage.

Thanks,
Nick

On Mon, 24 Sep 2018 01:08:20 +1000
Nicholas Piggin <npiggin@gmail.com> wrote:

> Fault paths like do_read_fault will install a Linux pte with the young
> bit clear. The CPU will fault again because the TLB has not been
> updated, this time a valid pte exists so handle_pte_fault will just
> set the young bit with ptep_set_access_flags, which flushes the TLB.
> 
> The TLB is flushed so the next attempt will go to the fast TLB handler
> which loads the TLB with the new Linux pte. The access then proceeds.
> 
> This design is fragile to depend on the young bit being clear after
> the initial Linux fault. A proposed core mm change to immediately set
> the young bit upon such a fault, results in ptep_set_access_flags not
> flushing the TLB because it finds no change to the pte. The spurious
> fault fix path only flushes the TLB if the access was a store. If it
> was a load, then this results in an infinite loop of page faults.
> 
> This change adds a TLB flush in update_mmu_cache, which removes that
> TLB entry upon the first fault. This will cause the fast TLB handler
> to load the new pte and avoid the Linux page fault entirely.
> 
> Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
> ---
>  arch/nios2/mm/cacheflush.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/arch/nios2/mm/cacheflush.c b/arch/nios2/mm/cacheflush.c
> index 506f6e1c86d5..d58e7e80dc0d 100644
> --- a/arch/nios2/mm/cacheflush.c
> +++ b/arch/nios2/mm/cacheflush.c
> @@ -204,6 +204,8 @@ void update_mmu_cache(struct vm_area_struct *vma,
>  	struct page *page;
>  	struct address_space *mapping;
>  
> +	flush_tlb_page(vma, address);
> +
>  	if (!pfn_valid(pfn))
>  		return;
>  
> -- 
> 2.18.0
>