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[2003:cb:c70d:1f00:7a4e:8f21:98db:baef]) by smtp.gmail.com with ESMTPSA id bi12-20020a05600c3d8c00b0041a959036f2sm7117510wmb.43.2024.04.24.02.38.52 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 24 Apr 2024 02:38:53 -0700 (PDT) Message-ID: <1af4fd61-7926-47c8-be45-833c0dbec08b@redhat.com> Date: Wed, 24 Apr 2024 11:38:51 +0200 Precedence: bulk X-Mailing-List: linux-trace-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v1 01/18] mm: allow for detecting underflows with page_mapcount() again To: linux-kernel@vger.kernel.org Cc: linux-mm@kvack.org, linux-doc@vger.kernel.org, cgroups@vger.kernel.org, linux-sh@vger.kernel.org, linux-trace-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, Andrew Morton , "Matthew Wilcox (Oracle)" , Peter Xu , Ryan Roberts , Yin Fengwei , Yang Shi , Zi Yan , Jonathan Corbet , Hugh Dickins , Yoshinori Sato , Rich Felker , John Paul Adrian Glaubitz , Chris Zankel , Max Filippov , Muchun Song , Miaohe Lin , Naoya Horiguchi , Richard Chang References: <20240409192301.907377-1-david@redhat.com> <20240409192301.907377-2-david@redhat.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 09.04.24 21:22, 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. > > We perform such underflow checks in zap_present_folio_ptes() and > zap_huge_pmd(), which would currently no longer trigger. > > Let's check against PAGE_MAPCOUNT_RESERVE instead by using > page_type_has_type(), like page_has_type() would, so we can still catch > some underflows. > > Fixes: 53277bcf126d ("mm: support page_mapcount() on page_has_type() pages") > Signed-off-by: David Hildenbrand > --- > include/linux/mm.h | 5 ++--- > 1 file changed, 2 insertions(+), 3 deletions(-) > > diff --git a/include/linux/mm.h b/include/linux/mm.h > index ef34cf54c14f..0fb8a40f82dd 100644 > --- a/include/linux/mm.h > +++ b/include/linux/mm.h > @@ -1229,11 +1229,10 @@ static inline void page_mapcount_reset(struct page *page) > */ > static inline int page_mapcount(struct page *page) > { > - int mapcount = atomic_read(&page->_mapcount) + 1; > + int mapcount = atomic_read(&page->_mapcount); > > /* Handle page_has_type() pages */ > - if (mapcount < 0) > - mapcount = 0; > + mapcount = page_type_has_type(mapcount) ? 0 : mapcount + 1; > if (unlikely(PageCompound(page))) > mapcount += folio_entire_mapcount(page_folio(page)); > From b49849001f3d2aad0af93cf2098065d7cbd9a959 Mon Sep 17 00:00:00 2001 From: David Hildenbrand Date: Wed, 24 Apr 2024 10:50:09 +0200 Subject: [PATCH] !fixup: mm: allow for detecting underflows with page_mapcount() again Let's make page_mapcount() slighly more efficient by inlining the page_type_has_type() check. Signed-off-by: David Hildenbrand --- include/linux/mm.h | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-) diff --git a/include/linux/mm.h b/include/linux/mm.h index dc33f8269fb52..cf700c5cdd58b 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -1229,10 +1229,11 @@ static inline void page_mapcount_reset(struct page *page) */ static inline int page_mapcount(struct page *page) { - int mapcount = atomic_read(&page->_mapcount); + int mapcount = atomic_read(&page->_mapcount) + 1; /* Handle page_has_type() pages */ - mapcount = page_type_has_type(mapcount) ? 0 : mapcount + 1; + if (mapcount < PAGE_MAPCOUNT_RESERVE + 1) + mapcount = 0; if (unlikely(PageCompound(page))) mapcount += folio_entire_mapcount(page_folio(page)); -- 2.44.0 -- Cheers, David / dhildenb