linux-kernel - Re: [PATCH] mm/gup: Fix follow_devmap

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Message-Id: <20231124112059.3519d6fdfe71f846f8bf726f@linux-foundation.org>
Date:   Fri, 24 Nov 2023 11:20:59 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Peter Xu <peterx@...hat.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Dan Williams <dan.j.williams@...el.com>,
        Mel Gorman <mgorman@...e.de>,
        Matthew Wilcox <willy@...radead.org>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.vnet.ibm.com>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH] mm/gup: Fix follow_devmap_p[mu]d() on page==NULL
 handling

On Thu, 23 Nov 2023 13:02:22 -0500 Peter Xu <peterx@...hat.com> wrote:

> This is a bug found not by any report but only by code observations.
> 
> When GUP sees a devpmd/devpud and if page==NULL is returned, it means a
> fault is probably required.  Here falling through when page==NULL can cause
> unexpected behavior.
> 

Well this is worrisome.  We aren't able to construct a test case to
demonstrate this bug?  Why is that?  Is it perhaps just dead code?