linux-kernel - Re: [PATCH v1 01/18] mm: allow for detecting underflows with page

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Date: Tue, 9 Apr 2024 22:42:29 +0100
From: Matthew Wilcox <willy@...radead.org>
To: David Hildenbrand <david@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linux-doc@...r.kernel.org, cgroups@...r.kernel.org,
	linux-sh@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Xu <peterx@...hat.com>, Ryan Roberts <ryan.roberts@....com>,
	Yin Fengwei <fengwei.yin@...el.com>, Yang Shi <shy828301@...il.com>,
	Zi Yan <ziy@...dia.com>, Jonathan Corbet <corbet@....net>,
	Hugh Dickins <hughd@...gle.com>,
	Yoshinori Sato <ysato@...rs.sourceforge.jp>,
	Rich Felker <dalias@...c.org>,
	John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>,
	Chris Zankel <chris@...kel.net>, Max Filippov <jcmvbkbc@...il.com>,
	Muchun Song <muchun.song@...ux.dev>,
	Miaohe Lin <linmiaohe@...wei.com>,
	Naoya Horiguchi <naoya.horiguchi@....com>,
	Richard Chang <richardycc@...gle.com>
Subject: Re: [PATCH v1 01/18] mm: allow for detecting underflows with
 page_mapcount() again

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.