[PATCH] ARM: mm: make UACCESS_WITH_MEMCPY huge page aware

[Nicolas Pitre]

On Tue, 24 Sep 2013, Steve Capper wrote:

> The memory pinning code in uaccess_with_memcpy.c does not check
> for HugeTLB or THP pmds, and will enter an infinite loop should
> a __copy_to_user or __clear_user occur against a huge page.
> 
> This patch adds detection code for huge pages to pin_page_for_write.
> As this code can be executed in a fast path it refers to the actual
> pmds rather than the vma. If a HugeTLB or THP is found (they have
> the same pmd representation on ARM), the page table spinlock is
> taken to prevent modification whilst the page is pinned.
> 
> On ARM, huge pages are only represented as pmds, thus no huge pud
> checks are performed. (For huge puds one would lock the page table
> in a similar manner as in the pmd case).
> 
> Two helper functions are introduced; pmd_thp_or_huge will check
> whether or not a page is huge or transparent huge (which have the
> same pmd layout on ARM), and pmd_hugewillfault will detect whether
> or not a page fault will occur on write to the page.
> 
> ---
> 
> Changes since second RFC:
>     * After some more thought about the potential impact on the
>       overall latency, I have decided not to widen the page mask
>       for huge pages.
> 
> Changes since first RFC:
>     * The page mask is widened for hugepages to reduce the number
>       of potential locks/unlocks.
>       (A knobbled /dev/zero with its latency reduction chunks
>       removed shows a 2x data rate boost with hugepages backing:
>       dd if=/dev/zero of=/dev/null bs=10M count=1024 )
> 
> ---
> 
> Signed-off-by: Steve Capper <steve.capper at linaro.org>

Be careful with your usage of "---" as git will strip anything below the 
first one for the commit log.  This means you should put your SOB above 
the first "---".

Also this would be good to capture the discussion about the performance 
implications of the different page mask sizes with test results in the 
commit log as well.

With the above, you may add:

Reviewed-by: nicolas Pitre <nico at linaro.org>



> ---
>  arch/arm/include/asm/pgtable-3level.h |  3 +++
>  arch/arm/lib/uaccess_with_memcpy.c    | 41 ++++++++++++++++++++++++++++++++---
>  2 files changed, 41 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm/include/asm/pgtable-3level.h b/arch/arm/include/asm/pgtable-3level.h
> index 5689c18..39c54cf 100644
> --- a/arch/arm/include/asm/pgtable-3level.h
> +++ b/arch/arm/include/asm/pgtable-3level.h
> @@ -206,6 +206,9 @@ static inline pmd_t *pmd_offset(pud_t *pud, unsigned long addr)
>  #define __HAVE_ARCH_PMD_WRITE
>  #define pmd_write(pmd)		(!(pmd_val(pmd) & PMD_SECT_RDONLY))
>  
> +#define pmd_hugewillfault(pmd)	(!pmd_young(pmd) || !pmd_write(pmd))
> +#define pmd_thp_or_huge(pmd)	(pmd_huge(pmd) || pmd_trans_huge(pmd))
> +
>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
>  #define pmd_trans_huge(pmd)	(pmd_val(pmd) && !(pmd_val(pmd) & PMD_TABLE_BIT))
>  #define pmd_trans_splitting(pmd) (pmd_val(pmd) & PMD_SECT_SPLITTING)
> diff --git a/arch/arm/lib/uaccess_with_memcpy.c b/arch/arm/lib/uaccess_with_memcpy.c
> index 025f742..3e58d71 100644
> --- a/arch/arm/lib/uaccess_with_memcpy.c
> +++ b/arch/arm/lib/uaccess_with_memcpy.c
> @@ -18,6 +18,7 @@
>  #include <linux/hardirq.h> /* for in_atomic() */
>  #include <linux/gfp.h>
>  #include <linux/highmem.h>
> +#include <linux/hugetlb.h>
>  #include <asm/current.h>
>  #include <asm/page.h>
>  
> @@ -40,7 +41,35 @@ pin_page_for_write(const void __user *_addr, pte_t **ptep, spinlock_t **ptlp)
>  		return 0;
>  
>  	pmd = pmd_offset(pud, addr);
> -	if (unlikely(pmd_none(*pmd) || pmd_bad(*pmd)))
> +	if (unlikely(pmd_none(*pmd)))
> +		return 0;
> +
> +	/*
> +	 * A pmd can be bad if it refers to a HugeTLB or THP page.
> +	 *
> +	 * Both THP and HugeTLB pages have the same pmd layout
> +	 * and should not be manipulated by the pte functions.
> +	 *
> +	 * Lock the page table for the destination and check
> +	 * to see that it's still huge and whether or not we will
> +	 * need to fault on write, or if we have a splitting THP.
> +	 */
> +	if (unlikely(pmd_thp_or_huge(*pmd))) {
> +		ptl = &current->mm->page_table_lock;
> +		spin_lock(ptl);
> +		if (unlikely(!pmd_thp_or_huge(*pmd)
> +			|| pmd_hugewillfault(*pmd)
> +			|| pmd_trans_splitting(*pmd))) {
> +			spin_unlock(ptl);
> +			return 0;
> +		}
> +
> +		*ptep = NULL;
> +		*ptlp = ptl;
> +		return 1;
> +	}
> +
> +	if (unlikely(pmd_bad(*pmd)))
>  		return 0;
>  
>  	pte = pte_offset_map_lock(current->mm, pmd, addr, &ptl);
> @@ -94,7 +123,10 @@ __copy_to_user_memcpy(void __user *to, const void *from, unsigned long n)
>  		from += tocopy;
>  		n -= tocopy;
>  
> -		pte_unmap_unlock(pte, ptl);
> +		if (pte)
> +			pte_unmap_unlock(pte, ptl);
> +		else
> +			spin_unlock(ptl);
>  	}
>  	if (!atomic)
>  		up_read(&current->mm->mmap_sem);
> @@ -147,7 +179,10 @@ __clear_user_memset(void __user *addr, unsigned long n)
>  		addr += tocopy;
>  		n -= tocopy;
>  
> -		pte_unmap_unlock(pte, ptl);
> +		if (pte)
> +			pte_unmap_unlock(pte, ptl);
> +		else
> +			spin_unlock(ptl);
>  	}
>  	up_read(&current->mm->mmap_sem);
>  
> -- 
> 1.8.1.4
>