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Date:   Sun, 19 Nov 2023 20:55:44 +0000
From:   Matthew Wilcox <willy@infradead.org>
To:     Kairui Song <kasong@tencent.com>
Cc:     linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
        "Huang, Ying" <ying.huang@intel.com>,
        David Hildenbrand <david@redhat.com>,
        Hugh Dickins <hughd@google.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@suse.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 01/24] mm/swap: fix a potential undefined behavior issue
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References: <20231119194740.94101-1-ryncsn@gmail.com>
 <20231119194740.94101-2-ryncsn@gmail.com>
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On Mon, Nov 20, 2023 at 03:47:17AM +0800, Kairui Song wrote:
> From: Kairui Song <kasong@tencent.com>
> 
> When folio is NULL, taking the address of its struct member is an
> undefined behavior, the UB is caused by applying -> operator
> to a pointer not pointing to any object. Although in practice this
> won't lead to a real issue, still better to fix it, also makes the
> code less error-prone, when folio is NULL, page is also NULL,
> instead of a meanless offset value.

Um, &folio->page is NULL if folio is NULL.  The offset of 'page' within
'folio' is 0.  By definition; and this will never change.