Re: [PATCH 1/6] fs: add read support for RWF_UNCACHED

[Guoqing Jiang <guoqing.jiang@cloud.ionos.com>]

On 12/17/19 3:39 PM, Jens Axboe wrote:
> If RWF_UNCACHED is set for io_uring (or preadv2(2)), we'll use private
> pages for the buffered reads. These pages will never be inserted into
> the page cache, and they are simply droped when we have done the copy at
> the end of IO.
>
> If pages in the read range are already in the page cache, then use those
> for just copying the data instead of starting IO on private pages.
>
> A previous solution used the page cache even for non-cached ranges, but
> the cost of doing so was too high. Removing nodes at the end is
> expensive, even with LRU bypass. On top of that, repeatedly
> instantiating new xarray nodes is very costly, as it needs to memset 576
> bytes of data, and freeing said nodes involve an RCU call per node as
> well. All that adds up, making uncached somewhat slower than O_DIRECT.
>
> With the current*solition*, we're basically at O_DIRECT levels of

Maybe it is 'solution' here.

Thanks,
Guoqing