linux-kernel - Re: [RFC PATCH 0/2] mm/gup: fix gup_fast with dynamic page table folding

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Message-ID: <20200902201355.GJ24045@ziepe.ca>
Date:   Wed, 2 Sep 2020 17:13:55 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Gerald Schaefer <gerald.schaefer@...ux.ibm.com>
Cc:     John Hubbard <jhubbard@...dia.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        linux-s390@...r.kernel.org, Heiko Carstens <hca@...ux.ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>
Subject: Re: [RFC PATCH 0/2] mm/gup: fix gup_fast with dynamic page table
 folding

On Wed, Sep 02, 2020 at 05:09:58PM +0200, Gerald Schaefer wrote:
> I guess we *could* assume that all the extra pXd_offset() calls and
> also the de-referencing would be optimized out by the compiler for other
> archs, but it is one example where my gut tells me that this might not
> be so trivial and w/o unwanted effects after all.

Assigning to a variable that is never read should be eliminated.. If
things are very complex then the pXX_offset function might need to be
marked with attribute pure, but I think this should be reliable?

Jason