Re: [PING] Avoid excessive buffer size in libio

[Carlos O'Donell <carlos at redhat dot com>]

On 11/29/2016 11:50 AM, Florian Weimer wrote:
> On 06/24/2016 05:35 PM, Florian Weimer wrote:
>> On 05/19/2016 04:57 PM, Florian Weimer wrote:
>>> On 03/31/2016 12:14 PM, Florian Weimer wrote:
>>>> On 03/18/2016 11:52 PM, Roland McGrath wrote:
>>>>
>>>>> Whatever the results, they would not IMHO be relevant here.
>>>>>
>>>>> POSIX specifies that st_blksize is the "preferred I/O block size for
>>>>> this
>>>>> object".  It's the kernel's responsibility to give userland good advice
>>>>> through this channel.  If there are common buggy kernels that give bad
>>>>> advice, that is a reason to apply upper and lower limits to the
>>>>> advice from
>>>>> the kernel.  But the expectation should be that the kernel gets
>>>>> fixed to
>>>>> give good advice, and the optimal thing to do with a good kernel is to
>>>>> follow its advice.
>>>>>
>>>>> Since the recommended use of st_blksize in this way is a standard user
>>>>> feature and not just what stdio's implementation happens to do, there
>>>>> is an
>>>>> argument to be made that the limiting of the value should be done in
>>>>> the
>>>>> *stat functions reported st_blksize values rather than in stdio's
>>>>> use of
>>>>> them.  (I'm ambivalent about this point.)
>>>>
>>>> That's a good point.  I'll try to get feedback from kernel file system
>>>> developers on this matter.
>>>
>>> I wasn't able to get any feedback.  Based on Rich's point about random
>>> I/O and Roland's earlier suggestion, I'm just capping the reported
>>> buffer size to 8192 in the attached patch.
>>
>> Ping?
>>
>>   <https://sourceware.org/ml/libc-alpha/2016-05/msg00428.html>
> 
> Ping?

Just to confirm the user can still call stat themselves and adjust the
buffer size up if they want?

If that's the case then this looks good to me and users have a way
to re-enable the old behaviour.

This kind of tuning can be changed at will from release to release
or patched by the distributions if they have problems with the change.

I don't see any real issue here, the kernel is returning "optimal" values
but this conflicts with what the user themselves want to be doing. Thus
providing a fixed cap for the default is fine (as long as the user can undo
the choice).

-- 
Cheers,
Carlos.