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

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Date:   Sat, 13 Oct 2018 14:19:57 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Christoph Hellwig <hch@...radead.org>
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 10/13/18 9:47 AM, Christoph Hellwig wrote:
> 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?

Not yet. I can go do that now. If you have any particular test suites, benchmarks,
or just programs to recommend, please let me know. So far, I see

   tools/testing/selftests/vm/gup_benchmark.c


-- 
thanks,
John Hubbard
NVIDIA