linux-kernel - Re: [PATCH v2 1/5] pmem, dax: clean up clear_pmem()

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Message-ID: <x49si52fx3k.fsf@segfault.boston.devel.redhat.com>
Date:	Thu, 22 Oct 2015 16:48:47 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Dan Williams <dan.j.williams@...el.com>
Cc:	axboe@...com, jack@...e.cz, linux-nvdimm@...1.01.org,
	Dave Hansen <dave.hansen@...ux.intel.com>, david@...morbit.com,
	linux-kernel@...r.kernel.org, hch@....de, akpm@...ux-foundation.org
Subject: Re: [PATCH v2 1/5] pmem, dax: clean up clear_pmem()

Dan Williams <dan.j.williams@...el.com> writes:

> Both, __dax_pmd_fault, and clear_pmem() were taking special steps to
> clear memory a page at a time to take advantage of non-temporal
> clear_page() implementations.  However, x86_64 does not use
> non-temporal instructions for clear_page(), and arch_clear_pmem() was
> always incurring the cost of __arch_wb_cache_pmem().
>
> Clean up the assumption that doing clear_pmem() a page at a time is more
> performant.

Wouldn't another solution be to actually use non-temporal stores?  Why
did you choose to punt?

Cheers,
Jeff

>
> Cc: Ross Zwisler <ross.zwisler@...ux.intel.com>
> Reported-by: Dave Hansen <dave.hansen@...ux.intel.com>
> Signed-off-by: Dan Williams <dan.j.williams@...el.com>
> ---
>  arch/x86/include/asm/pmem.h |    7 +------
>  fs/dax.c                    |    4 +---
>  2 files changed, 2 insertions(+), 9 deletions(-)
>
> diff --git a/arch/x86/include/asm/pmem.h b/arch/x86/include/asm/pmem.h
> index d8ce3ec816ab..1544fabcd7f9 100644
> --- a/arch/x86/include/asm/pmem.h
> +++ b/arch/x86/include/asm/pmem.h
> @@ -132,12 +132,7 @@ static inline void arch_clear_pmem(void __pmem *addr, size_t size)
>  {
>  	void *vaddr = (void __force *)addr;
>  
> -	/* TODO: implement the zeroing via non-temporal writes */
> -	if (size == PAGE_SIZE && ((unsigned long)vaddr & ~PAGE_MASK) == 0)
> -		clear_page(vaddr);
> -	else
> -		memset(vaddr, 0, size);
> -
> +	memset(vaddr, 0, size);
>  	__arch_wb_cache_pmem(vaddr, size);
>  }
>  
> diff --git a/fs/dax.c b/fs/dax.c
> index a86d3cc2b389..5dc33d788d50 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -623,9 +623,7 @@ int __dax_pmd_fault(struct vm_area_struct *vma, unsigned long address,
>  			goto fallback;
>  
>  		if (buffer_unwritten(&bh) || buffer_new(&bh)) {
> -			int i;
> -			for (i = 0; i < PTRS_PER_PMD; i++)
> -				clear_pmem(kaddr + i * PAGE_SIZE, PAGE_SIZE);
> +			clear_pmem(kaddr, HPAGE_SIZE);
>  			wmb_pmem();
>  			count_vm_event(PGMAJFAULT);
>  			mem_cgroup_count_vm_event(vma->vm_mm, PGMAJFAULT);
>
> _______________________________________________
> Linux-nvdimm mailing list
> Linux-nvdimm@...ts.01.org
> https://lists.01.org/mailman/listinfo/linux-nvdimm
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