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[149.14.88.26]) by smtp.gmail.com with ESMTPSA id d75a77b69052e-4b2b8de5fdesm85480771cf.31.2025.08.27.02.38.54 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 27 Aug 2025 02:38:57 -0700 (PDT) Message-ID: <66dac672-88f8-4101-9a66-382cd3794756@redhat.com> Date: Wed, 27 Aug 2025 11:38:53 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 2/2] mm/swap: add BUG_ON(folio==NULL) to folios_put_refs() To: Max Kellermann , akpm@linux-foundation.org, lorenzo.stoakes@oracle.com, ziy@nvidia.com, baolin.wang@linux.alibaba.com, Liam.Howlett@oracle.com, npache@redhat.com, ryan.roberts@arm.com, dev.jain@arm.com, baohua@kernel.org, shikemeng@huaweicloud.com, kasong@tencent.com, nphamcs@gmail.com, bhe@redhat.com, chrisl@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20250826231626.218675-1-max.kellermann@ionos.com> <20250826231626.218675-2-max.kellermann@ionos.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 27.08.25 01:16, Max Kellermann wrote: > It is not legal to have NULL pointers in a folio_batch. > > However, the Ceph code does exactly this, and a refactoring patch gone > wrong has exposed this to folios_put_refs(), see > https://lore.kernel.org/ceph-devel/aK4v548CId5GIKG1@swift.blarg.de/ > > I believe this should Oops instead of crashing due to NULL pointer > reference (guarded by is_huge_zero_folio(), which may silently hide > the bug). > > Signed-off-by: Max Kellermann > --- > mm/swap.c | 1 + > 1 file changed, 1 insertion(+) > > diff --git a/mm/swap.c b/mm/swap.c > index 3632dd061beb..07ccda00e7ee 100644 > --- a/mm/swap.c > +++ b/mm/swap.c > @@ -952,6 +952,7 @@ void folios_put_refs(struct folio_batch *folios, unsigned int *refs) > for (i = 0, j = 0; i < folios->nr; i++) { > struct folio *folio = folios->folios[i]; > unsigned int nr_refs = refs ? refs[i] : 1; > + BUG_ON(folio == NULL); We shouldn't be adding such checks into each and every function that expects a valid folio. The is_huge_zero_folio() is special because it does not crash immediately when the function is abused. So we don't want this (and in particular not a BUG_ON). -- Cheers David / dhildenb