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From: Lance Yang In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT X-Rspam-User: X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: D405540007 X-Stat-Signature: 3pioeexrapmrodh43dqmrtq5ujp6a7p4 X-HE-Tag: 1769048229-81049 X-HE-Meta: U2FsdGVkX18a9aG+VUKeu/SOoDmG5dv2xcHctzGXQC4+++mIb9Tx7qhcK+dUz8cTxidmx81loFI7JgfMnfSgmF5gc2Xu9SLuq9XGeY9lS/Q8rcPfEb+pMuWki0GPmPCEJse8inUxAUCcVTrhxzRQCWSlbm4g5tgGPKskbqLitI5XUHDnIuZWMJFY6eGvU4IVo+fvvarjpLiXQt6JqcX5SxjVEriTW47/IcTN3BvZBAcZ3v650dxEPHmDvWpYgJK09Pf/WProg3nWMShGcQexW5ffIt0KIeuAkQ6YrTnGHElVGeR3Q6B/I8v992slcPRc21n4HFLAnH1PrPLfJMKDHIeIgD4BX7J/CfO4yW1GvsxTi/xJQ7g+Wv4FZqeicQqj3XDpTKz2FpKJ/rlSlzmXY1UIYe4MKo5CJmMm3SBRW90KN+tlIZ72rLHajzz0/6TqDQpyM5+0aqlfrYlpsMFrk6avjRNQtIguHWbRsRH11QBdiR8iFOhYDMbVL++hhzd3ovcfCe+IuzP7VnA1oUYe2EH9WBMZg8PTdIY0ius9aXmZrhsBbRGl7LJMDXGjpCzC4VkvMaWhSdDXlitm6aMztcoj5tlat9f8rQZOq4MZrua7CiI0a6/rc8GI+ekdqGAzFoYr2y1IEnusBjYokTxA/59gG1WCbXHbCgve46sqwN9SdY5HvECWTUsgnOfWOciLDVtId09ll5KN4pT/l1yZqVzCo6oRyVJVzRFyyecf8zaPf10yVDuEWrFEn11r9N5SSxg/tdzKT2I7c+SpA26cS8YuSY6Nm6T2GTbBTw57kBNURH4AHX+qtDaR++nqiCMlBFpVGya7Fr5jz95z4kJSAqyZeQNjyTpmjxwVJWMWXM7EDcbJ8vWpRECdJuugIWvdR9nFkZKCXeek/N1Mlg4JqGE9aGjXhyK4Psd1YlymhjefND1QAgzd8HgCZTyEB2tJl8Jt2RtDMjKZKdAtzZ5 Quq3xJPa KCQl4Qp0d+Hwo8Th+kz2a9lplZW80kMS1aBf14jyymvf0IZtEXmOYrA8Civb9iYB8hlLmwMZ+dblQa2YGD3YC7ICvD14E1EcO0pbII2FQ5t3dHTu/GIMwIOj2Oaxy/Vh39WB2Avkg8QPUjGRn8cSWMS2Jkl9XQRMygNYowjSGMASrd1TfFJCbV+bilqX4ZE5HdDM0CcGBA0gYTFOM4ekMOvTykHl+Y9yEcUfAjcUi767IFZOyYE1bEaVsn4aIBZOtQqBs7t7F0E+qIwqem8LCRJFOEoMCT1S3bIT5CAkmfI3xPsyG1XbzD2p9+JNuKUmhBXGZacdu6m31IK0= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 2026/1/21 20:28, David Hildenbrand (Red Hat) wrote: > On 1/21/26 10:25, Lance Yang wrote: >> >> >> On 2026/1/21 16:13, zenghongling wrote: >>> The iterator variable 'folio' is swapped with 'prev' in the else >>> branch. Using 'folio' after swap() checks the potentially NULL >>> 'prev' value, not the original iterator value. >>> >>> Fix by moving folio_put() call before the swap operation in the >>> path where swap() occurs. >>> >>> Found by: >>> ./huge_memory.c:4225:6-11: ERROR: iterator variable bound on line >>> 4178 cannot be NULL >> > > Which tool did find that? A compiler? Looks like a false positive from coccinelle? The pattern is tricky to follow statically :) > >> Good catch! >> >> But which tree is your patch based on? >> >> Seems like that was already fixed in commit 776bde7caf80[1]. The >> whole thing deferred_split_scan() was refactored using folio_batch, >> so the buggy code with swap(folio, prev) is gone ... >> >> Ccing Muchun and Qi who fixed that. >> >> [1] >> https://lore.kernel.org/ >> all/59cb6b6fb5ffcff9d23b81890b252960139ad8e7.1762762324.git.zhengqi.arch@bytedance.com/ > > Right, in > > commit 776bde7caf80f6af72b087cafe7d9f607b14716d > Author: Muchun Song > Date:   Mon Nov 10 16:17:57 2025 +0800 > >     mm: thp: use folio_batch to handle THP splitting in > deferred_split_scan() > > > Which raises the question whether we would want to backport that patch > to stable kernels > if there was indeed a problem? > > > But: I don't immediately see the problem. > > If pref is NULL (and folio obviously !+NULL), we'll end up with >     * pref != NULL >     * folio == NULL > >     The "if (folio)" check will do nothing, because we defer the > freeing to the > >         if (prev) >             folio_put(prev); > >     later > > If pref is != NULL (and folio obviously !+NULL), we'll end up with >     * pref = NULL >     * folio = NULL > >     The if (folio) and if (prev) handling will care of it all. > > > So ... this pretty much looks like working as expected? Right! After reading through the old code, the swap() design is correct and working as intended, IIUC.