Subject: Re: [PATCH 3/5] powerpc/64s/hash: Fix fork() with 512TB process address space

From: "Aneesh Kumar K.V" <[email-masked]>
Date: Mon, 06 Nov 2017 16:14:51 +0530

Nicholas Piggin <[email-masked]> writes:

> Hash unconditionally resets the addr_limit to default (128TB) when
> the mm context is initialised. If a process has > 128TB mappings when
> it forks, the child will not get the 512TB addr_limit, so accesses to
> valid > 128TB mappings will fail in the child.
>
> Fix this by only resetting the addr_limit to default if it was 0. Non
> zero indicates it was duplicated from the parent (0 means exec()).
>

Reviewed-by: Aneesh Kumar K.V <[email-masked]>

> Cc: "Aneesh Kumar K.V" <[email-masked]>
> Fixes: f4ea6dcb08 ("powerpc/mm: Enable mappings above 128TB")
> Signed-off-by: Nicholas Piggin <[email-masked]>
> ---
>  arch/powerpc/mm/mmu_context_book3s64.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/arch/powerpc/mm/mmu_context_book3s64.c b/arch/powerpc/mm/mmu_context_book3s64.c
> index 05e15386d4cb..b94fb62e60fd 100644
> --- a/arch/powerpc/mm/mmu_context_book3s64.c
> +++ b/arch/powerpc/mm/mmu_context_book3s64.c
> @@ -93,11 +93,11 @@ static int hash__init_new_context(struct mm_struct *mm)
>  		return index;
>
>  	/*
> -	 * We do switch_slb() early in fork, even before we setup the
> -	 * mm->context.addr_limit. Default to max task size so that we copy the
> -	 * default values to paca which will help us to handle slb miss early.
> +	 * In the case of exec, use the default limit,
> +	 * otherwise inherit it from the mm we are duplicating.
>  	 */
> -	mm->context.addr_limit = DEFAULT_MAP_WINDOW_USER64;
> +	if (!mm->context.addr_limit)
> +		mm->context.addr_limit = DEFAULT_MAP_WINDOW_USER64;
>
>  	/*
>  	 * The old code would re-promote on fork, we don't do that when using
> -- 
> 2.15.0