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Date: Tue, 9 Apr 2024 22:42:29 +0100
From: Matthew Wilcox <willy@infradead.org>
To: David Hildenbrand <david@redhat.com>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	linux-doc@vger.kernel.org, cgroups@vger.kernel.org,
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	Andrew Morton <akpm@linux-foundation.org>,
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	Yin Fengwei <fengwei.yin@intel.com>, Yang Shi <shy828301@gmail.com>,
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	Yoshinori Sato <ysato@users.sourceforge.jp>,
	Rich Felker <dalias@libc.org>,
	John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>,
	Chris Zankel <chris@zankel.net>, Max Filippov <jcmvbkbc@gmail.com>,
	Muchun Song <muchun.song@linux.dev>,
	Miaohe Lin <linmiaohe@huawei.com>,
	Naoya Horiguchi <naoya.horiguchi@nec.com>,
	Richard Chang <richardycc@google.com>
Subject: Re: [PATCH v1 01/18] mm: allow for detecting underflows with
 page_mapcount() again
Message-ID: <ZhW2RQtKDvUrbyWA@casper.infradead.org>
References: <20240409192301.907377-1-david@redhat.com>
 <20240409192301.907377-2-david@redhat.com>
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In-Reply-To: <20240409192301.907377-2-david@redhat.com>

On Tue, Apr 09, 2024 at 09:22:44PM +0200, David Hildenbrand wrote:
> Commit 53277bcf126d ("mm: support page_mapcount() on page_has_type()
> pages") made it impossible to detect mapcount underflows by treating
> any negative raw mapcount value as a mapcount of 0.

Yes, but I don't think this is the right place to check for underflow.
We should be checking for that on modification, not on read.  I think
it's more important for page_mapcount() to be fast than a debugging aid.