Re: [RFC 1/2] arm: cacheflush syscall: process only pages that are in the memory

[Russell King - ARM Linux]

On Fri, Jan 26, 2018 at 12:14:40PM +0100, Marek Szyprowski wrote:
> glibc in calls cacheflush syscall on the whole textrels section of the
> relocated binaries. However, relocation usually doesn't touch all pages
> of that section, so not all of them are read to memory when calling this
> syscall. However flush_cache_user_range() function will unconditionally
> touch all pages from the provided range, resulting additional overhead
> related to reading all clean pages. Optimize this by calling
> flush_cache_user_range() only on the pages that are already in the
> memory.

What ensures that another CPU doesn't remove a page while we're
flushing it?  That will trigger a data abort, which will want to
take the mmap_sem, causing a deadlock.

> 
> Signed-off-by: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
> ---
>  arch/arm/kernel/traps.c | 25 +++++++++++++++++++------
>  1 file changed, 19 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/arm/kernel/traps.c b/arch/arm/kernel/traps.c
> index 5e3633c24e63..a5ec262ab30e 100644
> --- a/arch/arm/kernel/traps.c
> +++ b/arch/arm/kernel/traps.c
> @@ -564,23 +564,36 @@ static int bad_syscall(int n, struct pt_regs *regs)
>  static inline int
>  __do_cache_op(unsigned long start, unsigned long end)
>  {
> -	int ret;
> +	struct vm_area_struct *vma = NULL;
> +	int ret = 0;
>  
> +	down_read(&current->mm->mmap_sem);
>  	do {
>  		unsigned long chunk = min(PAGE_SIZE, end - start);
>  
> +		if (!vma || vma->vm_end <= start) {
> +			vma = find_vma(current->mm, start);
> +			if (!vma) {
> +				ret = -EFAULT;
> +				goto done;
> +			}
> +		}
> +
>  		if (fatal_signal_pending(current))
>  			return 0;
>  
> -		ret = flush_cache_user_range(start, start + chunk);
> -		if (ret)
> -			return ret;
> +		if (follow_page(vma, start, 0)) {
> +			ret = flush_cache_user_range(start, start + chunk);
> +			if (ret)
> +				goto done;
> +		}
>  
>  		cond_resched();
>  		start += chunk;
>  	} while (start < end);
> -
> -	return 0;
> +done:
> +	up_read(&current->mm->mmap_sem);
> +	return ret;
>  }
>  
>  static inline int
> -- 
> 2.15.0
> 

-- 
RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
FTTC broadband for 0.8mile line in suburbia: sync at 8.8Mbps down 630kbps up
According to speedtest.net: 8.21Mbps down 510kbps up