linux-kernel - Re: [PATCH v2] RDMA/umem: minor bug fix and cleanup in error handling paths

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Date:   Mon, 4 Mar 2019 20:53:38 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Ira Weiny <ira.weiny@...el.com>,
        Artemy Kovalyov <artemyko@...lanox.com>,
        "john.hubbard@...il.com" <john.hubbard@...il.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Doug Ledford <dledford@...hat.com>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>
Subject: Re: [PATCH v2] RDMA/umem: minor bug fix and cleanup in error
 handling paths

On Mon, Mar 04, 2019 at 03:11:05PM -0800, John Hubbard wrote:

> get_user_page(): increments page->_refcount by a large amount (1024)
> 
> put_user_page(): decrements page->_refcount by a large amount (1024)
> 
> ...and just stop doing the odd (to me) technique of incrementing once for
> each tail page. I cannot see any reason why that's actually required, as
> opposed to just "raise the page->_refcount enough to avoid losing the head
> page too soon".

I'd very much like to see this in the infiniband umem code - the extra
work and cost of touching every page in a huge page is very much
undesired.

Jason