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

[Christoph Hellwig <hch@infradead.org>]

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
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.

> 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.

> 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?