linux-kernel - Re: [PATCH 2/4] direct-io: make O

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Date:	Thu, 20 Aug 2009 09:19:04 -0400
From:	Trond Myklebust <trond.myklebust@....uio.no>
To:	Jens Axboe <jens.axboe@...cle.com>
Cc:	linux-kernel@...r.kernel.org, jeff@...zik.org,
	benh@...nel.crashing.org, htejun@...il.com, bzolnier@...il.com,
	alan@...rguk.ukuu.org.uk
Subject: Re: [PATCH 2/4] direct-io: make O_DIRECT IO path be page based

On Thu, 2009-08-20 at 12:17 +0200, Jens Axboe wrote:
> Currently we pass in the iovec array and let the O_DIRECT core
> handle the get_user_pages() business. This work, but it means that
> we can ever only use user pages for O_DIRECT.
> 
> Switch the aops->direct_IO() and below code to use page arrays
> instead, so that it doesn't make any assumptions about who the pages
> belong to. This works directly for all users but NFS, which just
> uses the same helper that the generic mapping read/write functions
> also call.
> 
> Signed-off-by: Jens Axboe <jens.axboe@...cle.com>
> ---
>  static ssize_t nfs_direct_write_schedule_segment(struct nfs_direct_req *dreq,
> -						 const struct iovec *iov,
> -						 loff_t pos, int sync)
> +						 struct dio_args *args,
> +						 int sync)
>  {
>  	struct nfs_open_context *ctx = dreq->ctx;
>  	struct inode *inode = ctx->path.dentry->d_inode;
> -	unsigned long user_addr = (unsigned long)iov->iov_base;
> -	size_t count = iov->iov_len;
> +	unsigned long user_addr = args->user_addr;
> +	size_t count = args->length;
>  	struct rpc_task *task;
>  	struct rpc_message msg = {
>  		.rpc_cred = ctx->cred,
> @@ -726,24 +702,8 @@ static ssize_t nfs_direct_write_schedule_segment(struct nfs_direct_req *dreq,
>  		if (unlikely(!data))
>  			break;
>  
> -		down_read(&current->mm->mmap_sem);
> -		result = get_user_pages(current, current->mm, user_addr,
> -					data->npages, 0, 0, data->pagevec, NULL);
> -		up_read(&current->mm->mmap_sem);
> -		if (result < 0) {
> -			nfs_writedata_free(data);
> -			break;
> -		}
> -		if ((unsigned)result < data->npages) {
> -			bytes = result * PAGE_SIZE;
> -			if (bytes <= pgbase) {
> -				nfs_direct_release_pages(data->pagevec, result);
> -				nfs_writedata_free(data);
> -				break;
> -			}
> -			bytes -= pgbase;
> -			data->npages = result;
> -		}
> +		data->pagevec = args->pages;
> +		data->npages = args->nr_segs;
>  
>  		get_dreq(dreq);
>  

This looks a bit odd. What guarantees that args->pages contain <= wsize
bytes? The server will not accept larger segments in a single RPC call.

Cheers
  Trond

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