Re: [PATCH v3 00/14] Pass data temperature information to SCSI disk devices

[Bart Van Assche <bvanassche@acm.org>]

On 10/18/23 17:33, Damien Le Moal wrote:
> On 10/19/23 04:34, Bart Van Assche wrote:
 >> On 10/18/23 12:09, Jens Axboe wrote:
>>> I'm also really against growing struct bio just for this. Why is patch 2
>>> not just using the ioprio field at least?
>>
>> Hmm ... shouldn't the bits in the ioprio field in struct bio have the
>> same meaning as in the ioprio fields used in interfaces between user
>> space and the kernel? Damien Le Moal asked me not to use any of the
>> ioprio bits passing data lifetime information from user space to the kernel.
> 
> I said so in the context that if lifetime is a per-inode property, then ioprio
> is the wrong interface since the ioprio API is per process or per IO. There is a
> mismatch.
> 
> One version of your patch series used fnctl() to set the lifetime per inode,
> which is fine, and then used the BIO ioprio to pass the lifetime down to the
> device driver. That is in theory a nice trick, but that creates conflicts with
> the userspace ioprio API if the user uses that at the same time.
> 
> So may be we should change bio ioprio from int to u16 and use the freedup u16
> for lifetime. With that, things are cleanly separated without growing struct bio.

Hmm ... I think that bi_ioprio has been 16 bits wide since the 
introduction of that data structure member in 2016?

>> Is it clear that the size of struct bio has not been changed because the
>> new bi_lifetime member fills a hole in struct bio?
> 
> When the struct is randomized, holes move or disappear. Don't count on that...

We should aim to maximize performance for users who do not use data 
structure layout randomization.

Additionally, I doubt that anyone is using full structure layout 
randomization for SCSI devices. No SCSI driver has any 
__no_randomize_layout / __randomize_layout annotations although I'm sure 
there are plenty of data structures in SCSI drivers for which the layout 
matters.

Thanks,

Bart.