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Date: Wed, 23 Nov 2016 08:29:54 -0800
From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
To: Al Viro <viro@ZenIV.linux.org.uk>
Cc: linux-kernel@vger.kernel.org, netdev@vger.kernel.org,
        Daniel Borkmann <daniel@iogearbox.net>,
        Martin KaFai Lau <kafai@fb.com>
Subject: Re: sendfile from 9p fs into af_alg
Message-ID: <20161123162952.GA48439@ast-mbp.thefacebook.com>
References: <20161123035827.GA43786@ast-mbp.thefacebook.com>
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 <20161123045558.GA43893@ast-mbp.thefacebook.com>
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 <20161123085809.GA46992@ast-mbp.thefacebook.com>
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On Wed, Nov 23, 2016 at 03:53:01PM +0000, Al Viro wrote:
> On Wed, Nov 23, 2016 at 12:58:11AM -0800, Alexei Starovoitov wrote:
> 
> > if I read it correctly 9p actually responded with 8192 bytes of requests...
> > whereas the file size was 9624.
> > For large file sizes (in megabytes) the difference between what
> > sendfile is reporting and actual file size can be 3x.
> > In the small file case (like above dump) it looks rounded to page size for some reason.
> 
> OK, I think I see one bug in there; could you check if this gets it back to
> normal?
> 
> diff --git a/fs/splice.c b/fs/splice.c
> index dcaf185..5a7750b 100644
> --- a/fs/splice.c
> +++ b/fs/splice.c
> @@ -408,7 +408,8 @@ static ssize_t default_file_splice_read(struct file *in, loff_t *ppos,
>  	if (res <= 0)
>  		return -ENOMEM;
>  
> -	nr_pages = res / PAGE_SIZE;
> +	BUG_ON(dummy);
> +	nr_pages = DIV_ROUND_UP(res, PAGE_SIZE);

Hooray. It fixed it :)
Feel free to add my
Tested-by: Alexei Starovoitov <ast@kernel.org>
when you submit it.
Thanks for fixing it so quickly!