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Thu, 12 Nov 2026 16:05:46 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id bcdNCNrj9WqGKQAAD6G6ig (envelope-from ); Thu, 12 Nov 2026 16:05:46 +0000 Date: Thu, 12 Nov 2026 16:05:40 +0000 From: Pedro Falcato To: David Hildenbrand Cc: Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , "Matthew Wilcox (Oracle)" , Jan Kara , Andrew Morton , Zi Yan , Baolin Wang , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif , Kiryl Shutsemau , Matthew Brost , Joshua Hahn , Byungchul Park , Gregory Price , Ying Huang , Alistair Popple , Peter Xu , Johannes Weiner , Song Liu , Rik van Riel , Miklos Szeredi , Alexander Viro , Christian Brauner , Hugh Dickins , Will Deacon , "Aneesh Kumar K.V" , Nick Piggin , Peter Zijlstra , Arnd Bergmann , Jason Gunthorpe , Kevin Tian , "Joerg Roedel (AMD)" , Robin Murphy , Steve Sistare , John Hubbard , "Jose A. Perez de Azpillaga" , Josef Bacik , "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman , Neal Cardwell , Kuniyuki Iwashima , Linus Torvalds , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, fuse-devel@lists.linux.dev, linux-arch@vger.kernel.org, iommu@lists.linux.dev, netdev@vger.kernel.org, stable@vger.kernel.org Subject: Re: [PATCH v3 6/9] RFC: net: limit folio refcount when copying skb frag references Message-ID: References: <20261110-refcount_overflow_stable-v3-0-318625d63a98@kernel.org> <20261110-refcount_overflow_stable-v3-6-318625d63a98@kernel.org> X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20261110-refcount_overflow_stable-v3-6-318625d63a98@kernel.org> X-Spam-Flag: NO X-Spam-Level: X-Spamd-Result: default: False [-11.80 / 50.00]; SUSE_INTERNAL_BYPASS(-9.00)[]; BAYES_HAM(-3.00)[100.00%]; SUSPICIOUS_RECIPS(1.50)[]; NEURAL_HAM_LONG(-1.00)[-1.000]; NEURAL_HAM_SHORT(-0.20)[-1.000]; MIME_GOOD(-0.10)[text/plain]; MISSING_XM_UA(0.00)[]; TAGGED_RCPT(0.00)[]; MIME_TRACE(0.00)[0:+]; RCVD_VIA_SMTP_AUTH(0.00)[]; ARC_NA(0.00)[]; RCVD_TLS_ALL(0.00)[]; DKIM_SIGNED(0.00)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; FROM_HAS_DN(0.00)[]; FREEMAIL_CC(0.00)[kernel.org,infradead.org,google.com,suse.com,suse.cz,linux-foundation.org,nvidia.com,linux.alibaba.com,linux.dev,arm.com,intel.com,gmail.com,sk.com,gourry.net,redhat.com,cmpxchg.org,fb.com,surriel.com,szeredi.hu,zeniv.linux.org.uk,arndb.de,ziepe.ca,8bytes.org,oracle.com,toxicpanda.com,davemloft.net,kvack.org,vger.kernel.org,lists.linux.dev]; TO_DN_SOME(0.00)[]; FROM_EQ_ENVFROM(0.00)[]; RCVD_COUNT_TWO(0.00)[2]; TO_MATCH_ENVRCPT_SOME(0.00)[]; RCPT_COUNT_GT_50(0.00)[62]; DBL_BLOCKED_OPENRESOLVER(0.00)[suse.de:email,imap1.dmz-prg2.suse.org:helo] X-Spam-Score: -11.80 On Tue, Nov 10, 2026 at 09:35:46PM +0100, David Hildenbrand (Arm) wrote: > Various networking features copy frag references of an skb, for example > when unsharing a cloned skb (AF_PACKET trimming a clone), when copying an > skb (nft dup) or when building NFQUEUE messages. This is an effective > way for multiplying folio references held by an skb, allowing for > exceeding folio refcount limits. > > This is not specific to large pagecache folios, but with them it is a > lot easier to reach the refcount limit, as page table mappings can cheaply > create many references. > > Let's introduce skb_frag_try_ref() and skb_frags_try_ref(), which use > try_get_page(), and use them in pskb_expand_head(), __pskb_copy_fclone(), > skb_zerocopy() and pskb_carve_inside_*(). All of these can already fail, > and their callers handle that. > > Fixes: 800d8c63b2e9 ("shmem: add huge pages support") > Fixes: 99cb0dbd47a1 ("mm,thp: add read-only THP support for (non-shmem) FS") > Fixes: 793917d997df ("mm/readahead: Add large folio readahead") > Cc: stable@vger.kernel.org > Signed-off-by: David Hildenbrand (Arm) Acked-by: Pedro Falcato For what it's worth, I tried thinking up a way to avoid all of this stuff. I know there are severe recursion limits that (should) avoid a large copy chain, but the issue is that, say you have a copy chain limit of 8, you can still create 2^31/8 skbs (each with their own reference), then clone each 8 times such that you get an overflow. Annoying. -- Pedro