Re: [PATCH v4 8/8] block: implement async write zero pages command

[Pavel Begunkov <asml.silence@gmail.com>]

On 9/12/24 10:26, Christoph Hellwig wrote:
> On Tue, Sep 10, 2024 at 09:10:34PM +0100, Pavel Begunkov wrote:
>> If we expect any error handling from the user space at all (we do),
>> it'll and have to be asynchronous, it's async commands and io_uring.
>> Asking the user to reissue a command in some form is normal.
> 
> The point is that pretty much all other errors are fatal, while this
> is a not supported for which we have a guaranteed to work kernel

Yes, and there will be an error indicating that it's not
supported, just like it'll return an error this io_uring
commands are not supported by a given kernel.

> fallback.  Kicking it off reuires a bit of work, but I'd rather have
> that in one place rather than applications that work on some hardware
> and not others.

There is nothing new in features that might be unsupported,
because of hardware or otherwise, it's giving control to the
userspace.

>> That's a shame, I agree, which is why I call it "presumably" faster,
>> but that actually gives more reasons why you might want this cmd
>> separately from write zeroes, considering the user might know
>> its hardware and the kernel doesn't try to choose which approach
>> faster.
> 
> But the kernel is the right place to make that decision, even if we
> aren't very smart about it right now.  Fanning that out to every
> single applications is a bad idea.

Apart that it will never happen

>> Users who know more about hw and e.g. prefer writes with 0 page as
>> per above. Users with lots of devices who care about pcie / memory
>> bandwidth, there is enough of those, they might want to do
>> something different like adjusting algorithms and throttling.
>> Better/easier testing, though of lesser importance.
>>
>> Those I made up just now on the spot, but the reporter did
>> specifically ask about some way to differentiate fallbacks.
> 
> Well, an optional nofallback flag would be in line with how we do
> that.  Do you have the original report to share somewhere?

Following with another flag "please do fallback", at which
point it doesn't make any sense when that can be done in
userspace.

-- 
Pavel Begunkov