Re: [PATCH 03/31] mm/pgtable: kmap_local_page() instead of kmap_atomic()

[Peter Xu <peterx@redhat.com>]

On Sun, May 21, 2023 at 09:52:31PM -0700, Hugh Dickins wrote:
> pte_offset_map() was still using kmap_atomic(): update it to the
> preferred kmap_local_page() before making further changes there, in case
> we need this as a bisection point; but I doubt it can cause any trouble.
> 
> Signed-off-by: Hugh Dickins <hughd@google.com>
> ---
>  include/linux/pgtable.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> index 8ec27fe69dc8..94235ff2706e 100644
> --- a/include/linux/pgtable.h
> +++ b/include/linux/pgtable.h
> @@ -96,9 +96,9 @@ static inline pte_t *pte_offset_kernel(pmd_t *pmd, unsigned long address)
>  
>  #if defined(CONFIG_HIGHPTE)
>  #define pte_offset_map(dir, address)				\
> -	((pte_t *)kmap_atomic(pmd_page(*(dir))) +		\
> +	((pte_t *)kmap_local_page(pmd_page(*(dir))) +		\
>  	 pte_index((address)))
> -#define pte_unmap(pte) kunmap_atomic((pte))
> +#define pte_unmap(pte) kunmap_local((pte))
>  #else
>  #define pte_offset_map(dir, address)	pte_offset_kernel((dir), (address))
>  #define pte_unmap(pte) ((void)(pte))	/* NOP */

(I think this could be a dumb question if this patch has been running there
 for years downstream, but still..)

I assume one major difference of using kmap_local() is page fault will not
be disabled, while kmap_atomic() will.

Meanwhile, pte_offset_map() is also used by pte_offset_map_lock(), which
means before this patch CONFIG_HIGHPTE systems will disable pgfault before
taking pgtable lock for it, while it will stop doing so after the change.

Then what happens if a page fault happens on the same pgtable lock range
that is already taken by the thread context?  What stops the deadlock from
happening?

Thanks,

-- 
Peter Xu