Re: [PATCH] /dev/zero: also implement ->read

Rasmus Villemoes
2020-09-07T06:51:04Z

On 07/09/2020 08.20, Christoph Hellwig wrote:
> On Mon, Sep 07, 2020 at 12:34:37AM +0200, Rasmus Villemoes wrote:
>> On 03/09/2020 17.59, Christoph Hellwig wrote:
>
>>> +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;
>>
>> Probably nobody really cares, but currently doing
>>
>> read(fd, &unmapped_page - 5, 123);
>>
>> returns 5, and those five bytes do get cleared; if I'm reading the above
>> right you'd return -EFAULT for that case.
>>
>>
>>> + cleared += chunk;
>>> + count -= chunk;
>>> +
>>> + if (signal_pending(current))
>>> + return cleared ? cleared : -ERESTARTSYS;
>>
>> I can't see how we can get here without 'cleared' being positive, so
>> this can just be 'return cleared' (and if you fix the above EFAULT case
>> to more accurately track how much got cleared, there's probably no
>> longer any code to be symmetric with anyway).
>
> Yeah, I'll fix these up and resend.
>

Actually, while you're micro-optimizing it, AFAIK VFS already handles
count==0, so you can avoid the initial branch and the last
cond_resched() by writing it something like

while (1) {
size_t chunk = min_t(size_t, count, PAGE_SIZE), c;
c = chunk - clear_user(buf + cleared, chunk);
if (unlikely(!c))
return cleared ?: -EFAULT;
cleared += c;
count -= c;
if (!count || signal_pending())
return cleared;
cond_resched();
}

For the dd test case with the default bs=512 that avoids cond_resched()
and signal_pending() altogether.

Rasmus