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

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Date:   Sat, 13 Oct 2018 09:47:41 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Dave Chinner <david@...morbit.com>,
        Matthew Wilcox <willy@...radead.org>,
        Michal Hocko <mhocko@...nel.org>,
        Christopher Lameter <cl@...ux.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Dan Williams <dan.j.williams@...el.com>,
        Jan Kara <jack@...e.cz>, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-rdma <linux-rdma@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 4/6] mm: introduce page->dma_pinned_flags, _count

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?