bug#26311: dd support for POSIX_FADV_NOREUSE etc.

[Pádraig Brady]

On 30/03/17 10:55, Paul Eggert wrote:
> On 03/30/2017 09:30 AM, Pádraig Brady wrote:
>> Yes maybe, though the low level meanings of these flags are
>> a bit confusing and I'm not sure how consistent they are over kernel versions
> 
> Nor I. Still, 'dd' to some extent is supposed to be a low-level program 
> that lets one directly access syscall flags, so following that design 
> philosophy we should just pass the flags through and let users 
> experiment with them.
> 
>>
>>> Dumb question: why does dd iflag=nocache check the return value of
>>> posix_fadvise? The fadvise function ignores the return value and says
>>> why; do these reasons not apply to 'dd'?
>> Well for dd it's a request, which for other utils it's a performance 
>> advisement.
>> As stated in the info docs, for dd it's only significant in the special case
>> of dropping cache for the whole file:
>>
>>    $ : | dd iflag=nocache count=0 status=none
>>    dd: failed to discard cache for: 'standard input': Illegal seek
> 
> Yes, and that was what I was asking about. iflag=nocache is just advice 
> which the kernel is not obliged to follow even when the syscall succeeds 
> (and conversely, it's advice which the kernel might decided to follow 
> even when the syscall fails!). So why bother to report the syscall 
> failure in this particular case?
> 
> It's no big deal. More of a philosophical question, really.

Yes I'm not adamant on this behaviour, but my thinking
was with count=0 iflag=nocache, you are _only_ asking
for cache invalidation, so dd has more leeway to error
in this case (even though the only effective error reported
is EPIPE).  With other dd options, you could be combining
the nocaching directive with other operations and so it
would be ok for the nocache advice failure to be ignored.

cheers,
Pádraig