Re: [PATCH 4/6] mm: introduce page->dma_pinned_flags, _count

[Christoph Hellwig]

On Sat, Oct 13, 2018 at 12:34:12AM -0700, John Hubbard wrote:
> In patch 6/6, pin_page_for_dma(), which is called at the end of get_user_pages(),
> unceremoniously rips the pages out of the LRU, as a prerequisite to using
> either of the page->dma_pinned_* fields. 
> 
> The idea is that LRU is not especially useful for this situation anyway,
> so we'll just make it one or the other: either a page is dma-pinned, and
> just hanging out doing RDMA most likely (and LRU is less meaningful during that
> time), or it's possibly on an LRU list.

Have you done any benchmarking what this does to direct I/O performance,
especially for small I/O directly to a (fast) block device?