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Subject: Re: [PATCH] /dev/zero: also implement ->read
To:     Christoph Hellwig <hch@lst.de>, arnd@arndb.de,
        gregkh@linuxfoundation.org
Cc:     linux-kernel@vger.kernel.org
References: <20200903155922.1111551-1-hch@lst.de>
From:   Christophe Leroy <christophe.leroy@csgroup.eu>
Message-ID: <1bc34841-f1a3-8a9b-cb48-10930ec55d71@csgroup.eu>
Date:   Thu, 3 Sep 2020 19:51:07 +0200
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Le 03/09/2020 à 17:59, Christoph Hellwig a écrit :
> Christophe reported a major speedup due to avoiding the iov_iter
> overhead, so just add this trivial function.  Note that /dev/zero
> already implements both an iter and non-iter writes so this just
> makes it more symmetric.
> 
> Christophe Leroy <christophe.leroy@csgroup.eu>
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
>   drivers/char/mem.c | 22 ++++++++++++++++++++++
>   1 file changed, 22 insertions(+)
> 
> diff --git a/drivers/char/mem.c b/drivers/char/mem.c
> index abd4ffdc8cdebc..1dc99ab158457a 100644
> --- a/drivers/char/mem.c
> +++ b/drivers/char/mem.c
> @@ -726,6 +726,27 @@ static ssize_t read_iter_zero(struct kiocb *iocb, struct iov_iter *iter)
>   	return written;
>   }
>   
> +static ssize_t read_zero(struct file *file, char __user *buf,
> +			 size_t count, loff_t *ppos)
> +{
> +	size_t cleared = 0;
> +
> +	while (count) {
> +		size_t chunk = min_t(size_t, count, PAGE_SIZE);
> +
> +		if (clear_user(buf + cleared, chunk))
> +			return cleared ? cleared : -EFAULT;
> +		cleared += chunk;
> +		count -= chunk;
> +
> +		if (signal_pending(current))
> +			return cleared ? cleared : -ERESTARTSYS;
> +		cond_resched();
> +	}
> +
> +	return cleared;
> +}
> +
>   static int mmap_zero(struct file *file, struct vm_area_struct *vma)
>   {
>   #ifndef CONFIG_MMU
> @@ -921,6 +942,7 @@ static const struct file_operations zero_fops = {
>   	.llseek		= zero_lseek,
>   	.write		= write_zero,
>   	.read_iter	= read_iter_zero,
> +	.read		= read_zero,

Wondering if .read should be before .write, so that we get in order 
read, write, read_iter, write_iter.

>   	.write_iter	= write_iter_zero,
>   	.mmap		= mmap_zero,
>   	.get_unmapped_area = get_unmapped_area_zero,
> 

Christophe