linux-kernel - Re: [PATCH 4/7] O_NONBLOCK flag for readv2/preadv2

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Date:	Tue, 16 Sep 2014 15:44:13 -0400
From:	Milosz Tanski <milosz@...in.com>
To:	Jeff Moyer <jmoyer@...hat.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Christoph Hellwig <hch@...radead.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	linux-aio@...ck.org, Mel Gorman <mgorman@...e.de>,
	Volker Lendecke <Volker.Lendecke@...net.de>,
	Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH 4/7] O_NONBLOCK flag for readv2/preadv2

On Tue, Sep 16, 2014 at 3:19 PM, Jeff Moyer <jmoyer@...hat.com> wrote:
> Milosz Tanski <milosz@...in.com> writes:
>
>> Filesystems that generic_file_read_iter will not be allowed to perform
>> non-blocking reads. This only will read data if it's in the page cache and if
>> there is no page error (causing a re-read).
>>
>> Signed-off-by: Milosz Tanski <milosz@...in.com>
>
>> @@ -1662,6 +1676,10 @@ no_cached_page:
>>                       goto out;
>>               }
>>               goto readpage;
>> +
>> +would_block:
>> +             error = -EAGAIN;
>> +             goto out;
>>       }
>
> Why did you put the wouldblock label inside the loop?  That should be
> pushed down to just above out, and then you can get rid of the goto.

When I put the code outside the loop it actually looked worse (imo):

}

goto out;

would_block:
error = -EAGAIN;

out:
...

>
> Other than that, it looks like you put the check in all the right places
> in that function.
>
>>  out:
>> @@ -1697,6 +1715,9 @@ generic_file_read_iter(struct kiocb *iocb, struct iov_iter *iter, int flags)
>>               size_t count = iov_iter_count(iter);
>>               loff_t size;
>>
>> +             if (flags & O_NONBLOCK)
>> +                     return -EAGAIN;
>> +
>
> If a program is attempting non-blocking reads on a file opened with
> O_DIRECT, I think returning -EAGAIN is very misleading.  Better to
> return -EINVAL in this case, and maybe check that earlier in the stack?

Point taken and I can fix this for the next version further up the
stack. A longer term question is how the flags the file is open with
interact with the read/write flags ... since I imagine folks will want
to add other flags (like force a SYNC write).

>
> Cheers,
> Jeff

-- 
Milosz Tanski
CTO
16 East 34th Street, 15th floor
New York, NY 10016

p: 646-253-9055
e: milosz@...in.com
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