Re: [PATCH v5 1/2] Return bytes transferred for partial direct I/O

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> On Jan 20, 2018, at 8:07 PM, Jens Axboe <axboe@xxxxxxxxx> wrote:
> 
> On 1/20/18 7:23 PM, Goldwyn Rodrigues wrote:
>> 
>> 
>> On 01/20/2018 08:11 PM, Andi Kleen wrote:
>>> 
>>> It's likely there's a lot of code in user space that does
>>> 
>>>     if (write(..., N) < 0) handle error
>>> 
>>> With your change it would need to be
>>> 
>>>     if (write(..., N) != N) handle error
>>> 
>>> How much code is actually doing that?
>>> 
>>> I can understand it fixes your artifical test suite, but it seems to me your
>>> change has a high potential to break a lot of existing user code
>>> in subtle ways. So it seems to be a bad idea.
>> 
>> 
>> Quoting 'man 2 write':
>> 
>> RETURN VALUE
>> On  success,  the  number  of bytes written is returned (zero indicates
>> nothing was written).  It is not an error if  this  number  is  smaller
>> than the number of bytes requested; this may happen for example because
>> the disk device was filled.  See also NOTES.
> 
> You can quote as much man page as you want - Andi is well aware of how
> read/write system call works, as I'm sure all of us are, that is not the
> issue. The issue is that there are potentially LOTS of applications out
> there that do not check for short writes, they do exactly what Andi
> speculated above. If you break it with this change, it doesn't matter
> what's in the man page. What matters is previous behavior, and that
> you are breaking user space. At that point nobody cares what's in the
> man page.

Applications that don't handle partial writes are already broken with
buffered I/O, this change doesn't really make them more broken.

Cheers, Andreas





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