linux-kernel - Re: [PATCH 10/10] powerpc: remove address space overrides using set

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Date:   Thu, 3 Sep 2020 10:55:52 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Christoph Hellwig <hch@....de>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        Michael Ellerman <mpe@...erman.id.au>,
        the arch/x86 maintainers <x86@...nel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Kees Cook <keescook@...omium.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 10/10] powerpc: remove address space overrides using
 set_fs()



Le 03/09/2020 à 09:11, Christoph Hellwig a écrit :
> 
> Except that we do not actually have such a patch.  For normal user
> writes we only use ->write_iter if ->write is not present.  But what
> shows up in the profile is that /dev/zero only has a read_iter op and
> not a normal read.  I've added a patch below that implements a normal
> read which might help a tad with this workload, but should not be part
> of a regression.
> 

With that patch below, throughput is 113.5MB/s (instead of 99.9MB/s).
So a 14% improvement. That's not bad.

Christophe

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