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

[Jens Axboe <axboe@kernel.dk>]

On 12/17/19 8:16 AM, Guoqing Jiang wrote:
> 
> 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.

Indeed, fixed up, thanks.

-- 
Jens Axboe