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Wed, 08 Jul 2026 11:36:38 -0700 (PDT) Date: Wed, 8 Jul 2026 14:36:31 -0400 From: Johannes Weiner To: Michael Hicklen Cc: linux-ext4@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, tytso@mit.edu, vbabka@suse.cz Subject: Re: kernel NULL pointer dereference in drain_obj_stock during ext4 e4defrag Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspam-User: X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: 1B85BC0004 X-Stat-Signature: zwukz74pmdhrrozma5xw1no1ofzx8oo9 X-HE-Tag: 1783535799-513705 X-HE-Meta: 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 yf3XjSPM RD7DpyXw2G2rbul8fmPJudNlgs5fYcOK4or8LVXPtfQiKqanS2cD3amW+eOHt4XwA39wgHj6lqSangxN2qVyU9FYmBaSGwmLyPX45lGIfexZypkCYDtyO0fTCn5Eu+oeCom587vlSllus4nSf1qR495FQSmZFapxn7qk6w9nLrKPOAQuPF9Zmw0TzmGKLhTwl2/s5RvZAiVJeZXMoiHm1pTJpizUIbI3siMVgzDpq/3e9gJGPGloIDmRmZYB32Z0fSFQrfmjy66QL1g+4RCSm4rvRQwAwFXK1kKn5r1q/77bNs8+7kswvnGTpzi9FlHyv9bu0CCP3cs1/+2/+r4dN9OAXEal/PzElCd4VI47ss9G6L0a4/Q+uOdb3dagoS9LRyico12lWhejpVRtQYikSBKO2dHVrR3hCmR8jUbsyD4BkeskOKRiuirBAHCTZzzbZEbdUEovgXwHqDFPt6k08xOXjUhqzvtIqzOvee9fKr5s0mkydUkyoijTVsyvRA0mv/wocL8Kw+9IcZm03jiDsveipAg== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Hello Michael, On Tue, Jul 07, 2026 at 06:51:14PM -0500, Michael Hicklen wrote: > Kernel: 7.1.2-arch3-1 > Arch: x86_64 > Hardware: MSI MS-7C95 / PRO B550M-VC WIFI > CPU: AMD (kvm_amd loaded) > > Workload: > > 4 concurrent e4defrag processes operating on 4 separate ext4 > filesystems on 4 separate HDDs. > > The crash happens during memory reclaim while ext4_move_extents() is running. > > First oops: > > BUG: kernel NULL pointer dereference, address: 0000000000000676 > #PF: supervisor read access in kernel mode > > RIP: drain_obj_stock+0x175/0x310 Thanks for the report. The trace and register state suggests that we have the stock pointing to a dead objcg. Through this mess of container_of(), you basically have to read the lines in reverse: drain_obj_stock() mod_objcg_mlstate(old = stock->cached_objcg, stock->cached_pgdat, ...) rcu_read_lock() memcg = obj_cgroup_memcg(old) // objcg->memcg: stale value lruvec = mem_cgroup_lruvec(memcg, pgdat) // memcg->nodeinfo[nid]: stale value mod_memcg_lruvec_state(lruvec, ...) pn = container_of(lruvec, ...) memcg = pn->memcg // freed pn, first 8 bytes now contain 6 pn = memcg->nodeinfo[pgdat->node_id] // <<< faults: 6 + 0x670 = 0x676 Michael, do you have the full dmesg available from this machine? The oops you included already include taint flags for previous crashes and warnings. There is an interleaved Code: block (twice) that disassembles to what looks like an obj_cgroup_get() encountering a percpu_count_ptr of 0, which further suggests use-after-free of the objcg.