Re: [dm-devel] [PATCH 0/6] dax poison recovery with RWF_RECOVERY_DATA flag

Pavel Begunkov
2021-10-29T19:25:03Z

On 10/29/21 17:57, Darrick J. Wong wrote:
> On Fri, Oct 29, 2021 at 12:46:14PM +0100, Pavel Begunkov wrote:
>> On 10/28/21 23:59, Dave Chinner wrote:
>> [...]
>>>>> Well, my point is doing recovery from bit errors is by definition not
>>>>> the fast path. Which is why I'd rather keep it away from the pmem
>>>>> read/write fast path, which also happens to be the (much more important)
>>>>> non-pmem read/write path.
>>>>
>>>> The trouble is, we really /do/ want to be able to (re)write the failed
>>>> area, and we probably want to try to read whatever we can. Those are
>>>> reads and writes, not {pre,f}allocation activities. This is where Dave
>>>> and I arrived at a month ago.
>>>>
>>>> Unless you'd be ok with a second IO path for recovery where we're
>>>> allowed to be slow? That would probably have the same user interface
>>>> flag, just a different path into the pmem driver.
>>>
>>> I just don't see how 4 single line branches to propage RWF_RECOVERY
>>> down to the hardware is in any way an imposition on the fast path.
>>> It's no different for passing RWF_HIPRI down to the hardware *in the
>>> fast path* so that the IO runs the hardware in polling mode because
>>> it's faster for some hardware.
>>
>> Not particularly about this flag, but it is expensive. Surely looks
>> cheap when it's just one feature, but there are dozens of them with
>> limited applicability, default config kernels are already sluggish
>> when it comes to really fast devices and it's not getting better.
>> Also, pretty often every of them will add a bunch of extra checks
>> to fix something of whatever it would be.
>
> So we can't have data recovery because moving fast the only goal?

That's not what was said and you missed the point, which was in
the rest of the message.

>
> That's so meta.
>
> --D
>
>> So let's add a bit of pragmatism to the picture, if there is just one
>> user of a feature but it adds overhead for millions of machines that
>> won't ever use it, it's expensive.
>>
>> This one doesn't spill yet into paths I care about, but in general
>> it'd be great if we start thinking more about such stuff instead of
>> throwing yet another if into the path, e.g. by shifting the overhead
>> from linear to a constant for cases that don't use it, for instance
>> with callbacks or bit masks.
>>
>>> IOWs, saying that we shouldn't implement RWF_RECOVERY because it
>>> adds a handful of branches the fast path is like saying that we
>>> shouldn't implement RWF_HIPRI because it slows down the fast path
>>> for non-polled IO....
>>>
>>> Just factor the actual recovery operations out into a separate
>>> function like:
>>
>> --
>> Pavel Begunkov

--
Pavel Begunkov