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Date: Sat, 13 Oct 2018 09:47:41 -0700
From: Christoph Hellwig <hch@infradead.org>
Subject: Re: [PATCH 4/6] mm: introduce page->dma_pinned_flags, _count
Message-ID: <20181013164740.GA6593@infradead.org>
References: <20181012060014.10242-1-jhubbard@nvidia.com>
 <20181012060014.10242-5-jhubbard@nvidia.com>
 <20181013035516.GA18822@dastard>
 <7c2e3b54-0b1d-6726-a508-804ef8620cfd@nvidia.com>
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To: John Hubbard <jhubbard@nvidia.com>
Cc: Dave Chinner <david@fromorbit.com>, Matthew Wilcox <willy@infradead.org>, Michal Hocko <mhocko@kernel.org>, Christopher Lameter <cl@linux.com>, Jason Gunthorpe <jgg@ziepe.ca>, Dan Williams <dan.j.williams@intel.com>, Jan Kara <jack@suse.cz>, linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>, LKML <linux-kernel@vger.kernel.org>, linux-rdma <linux-rdma@vger.kernel.org>, linux-fsdevel@vger.kernel.org

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?