Re: [PATCHSET v3 0/5] Support for RWF_UNCACHED

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On 7 Jan 2020, at 12:42, Christoph Hellwig wrote:

> On Fri, Dec 13, 2019 at 01:32:10AM +0000, Chris Mason wrote:
>> They just have different tradeoffs.  O_DIRECT actively blows away 
>> caches
>> and can also force writes during reads, making RWF_UNCACHED a more
>> natural fit for some applications.  There are fewer surprises, and 
>> some
>> services are willing to pay for flexibility with a memcpy.  In 
>> general,
>> they still want to do some cache management because it reduces p90+
>> latencies across the board, and gives them more control over which 
>> pages
>> stay in cache.
>
> We can always have a variant of O_DIRECT that doesn't do that and
> instead check if data was in the cache and then also copy / from to
> it in that case.  I need some time to actually look through this 
> series,
> so it might be pretty similar to the implementation, but if defined
> the right way it could be concurrent for at least the fast path of no
> cached pages.

Yeah, I really do think we can end up with a fairly unified solution 
through iomap:

* Allowing concurrent writes (xfs DIO does this now)
* Optionally doing zero copy if alignment is good (btrfs DIO does this 
now)
* Optionally tossing pages at the end (requires a separate syscall now)
* Supporting aio via io_uring

We could just call this O_DIRECT, but I like RWF_UNCACHED as a way to 
avoid surprises for people that know and love the existing O_DIRECT 
semantics.

-chris




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