linux-kernel - Re: [PATCH v2 2/6] dax: Use copy_from_iter

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Date:	Sun, 05 Jul 2015 16:11:07 +0300
From:	Boaz Harrosh <boaz@...xistor.com>
To:	Matthew Wilcox <matthew.r.wilcox@...el.com>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	Alexander Viro <viro@...iv.linux.org.uk>
CC:	Matthew Wilcox <willy@...ux.intel.com>
Subject: Re: [PATCH v2 2/6] dax: Use copy_from_iter_nocache

On 07/03/2015 05:40 PM, Matthew Wilcox wrote:
> From: Matthew Wilcox <willy@...ux.intel.com>
> 
> When userspace does a write, there's no need for the written data to
> pollute the CPU cache.  This matches the original XIP code.
> 
> Signed-off-by: Matthew Wilcox <willy@...ux.intel.com>
> ---
>  fs/dax.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/dax.c b/fs/dax.c
> index 99b5fbc..eaa9e06 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -155,7 +155,7 @@ static ssize_t dax_io(struct inode *inode, struct iov_iter *iter,
>  		}
>  
>  		if (iov_iter_rw(iter) == WRITE)
> -			len = copy_from_iter(addr, max - pos, iter);
> +			len = copy_from_iter_nocache(addr, max - pos, iter);
>  		else if (!hole)
>  			len = copy_to_iter(addr, max - pos, iter);
>  		else
> 

With the current ioremap_nocache at pmem none of this matters for pmem.

For brd yes, so We've been conducting some measurements and regular ext4
(no DAX) benchmark gives 6-16% increase in performance with this above.
And DAX is almost x2 then no DAX.
Is why the network guys been using this for a long time. So I'd say this
is a good default for any page-cache writes. (Think about it it makes sense,
we will 95% of the time flush these to real memory before DMA)

For pmem with any sane cached mapping (We use page-stuct-pmem actually)
Then DAX, for it to actually work (persist) with pmem, needs this:

static size_t copy_from_iter_nt(void *addr, size_t bytes, struct iov_iter *ii)
{
	size_t ret = copy_from_iter_nocache(addr, bytes, ii);

	if (unlikely((ii->type & ITER_BVEC) || (ii->type & ITER_KVEC))) {
		/* FIXME: copy_from_iter_nocache did regular copy for Kernel
		 * buffers (BVEC or KVEC). Before we fix it do cl_flush
		 * for now.
		 */
		cl_flush(addr, bytes, false);
	} else {
		/* copy_from_iter_nocache only persists in 8-byte aligned words.
		 * Lets persist remaining unaligned edges.
		 */
		if (unlikely((ulong)addr & 0x7))
			cl_flush(addr, 1, false);
		if (unlikely((ulong)(addr + bytes) & 0x7))
			cl_flush((addr + bytes), 1, false);
	}

	return ret;
}

This is based on an not-in-kernel cl_flush().

The first part FIXME above could be fixed with Dan's memcpy_persistent() patches

Cheers
Boaz

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