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a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1777070961; bh=1l62ftx9rkfVGFVp1ptfxZUhOlbilrk3qypY8gQ/oic=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=HZKfSCGhqeT/Hgx9B3tULabhTtRaySMTyinTdk2FdWIyDC7t40ZprQtYh7JCySR8K 2wPYbEpNGkMwi7xDLMFjRV7dLQE7LLJwIlddQVn2FECNPdx1ldPqq5AgkzujbivYJ8 Mmz7sYngCWxENghv4T3MuDiar1qocWgKK31hWtO8PRYdepJ5+PP6Vc6AvP+Ahpwe+A 810t9AQCNij/dDbk0PuUYbz+cz8ob4pqtii8uFpeeHB1S0PuoufEX743NNQ0PxGGZs FzoOxw1GEO+PMV0eWGhlJ3prsncgxpRP4l3wgVbN8c7DOAFK7o5HiCWEyGgn+z0QSk tJuZiIi557lmA== Date: Fri, 24 Apr 2026 15:49:19 -0700 From: Minchan Kim To: Michal Hocko Cc: akpm@linux-foundation.org, hca@linux.ibm.com, linux-s390@vger.kernel.org, david@kernel.org, brauner@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, surenb@google.com, timmurray@google.com Subject: Re: [PATCH v1 3/3] mm: process_mrelease: introduce PROCESS_MRELEASE_REAP_KILL flag Message-ID: References: <20260421230239.172582-1-minchan@kernel.org> <20260421230239.172582-4-minchan@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Stat-Signature: xtnc3q89nykcrme1pgqw6w1f67h848is X-Rspam-User: X-Rspamd-Queue-Id: F22ED20005 X-Rspamd-Server: rspam07 X-HE-Tag: 1777070961-869414 X-HE-Meta: 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 MUUA5xEt G063xtb1bG2n2BdY7Aj71VTJSLsmI3TAw6fbxxjuzBxDPUQSKfSirTVDtY70p0DUnAHZUKIPozgl6WTatRywqu2VrQmIO7FdXE2160NCTbnII7S9idWZsbWUydAdjetj8hlkrdBv6Mmni6IZvh+nzUzXQOOss5luqRd3LPmSOjcFuk5yUgYgec8HkvsJ0sxACuF33gInCXPDe7bBuOQSBRr4XG/LHUNE+U15q5dKQ8gxqTKTTkldGPafMHpCZGi9AbI0a2JY7LuqiaDo31RCGnQRt5J5QFNEmQ5SekUceYu9WVFDMAPmI94F3yQzrCcv67AmEWJlVayMPY/eKSs1LYKhxzHo+Tj/0qg859ik1Q7yzZE0= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Fri, Apr 24, 2026 at 09:57:20AM +0200, Michal Hocko wrote: > On Tue 21-04-26 16:02:39, Minchan Kim wrote: > > Currently, process_mrelease() requires userspace to send a SIGKILL signal > > prior to the call. This separation introduces a scheduling race window > > where the victim task may receive the signal and enter the exit path > > before the reaper can invoke process_mrelease(). > > > > When the victim enters the exit path (do_exit -> exit_mm), it clears its > > task->mm immediately. This causes process_mrelease() to fail with -ESRCH, > > leaving the actual address space teardown (exit_mmap) to be deferred until > > the mm's reference count drops to zero. In Android, arbitrary reference counts > > (e.g., async I/O, reading /proc//cmdline, or various other remote > > VM accesses) frequently delay this teardown indefinitely, defeating the > > purpose of expedited reclamation. > > > > This delay keeps memory pressure high, forcing the system to unnecessarily > > kill additional innocent background apps before the memory from the first > > victim is recovered. > > Thanks, this makes the motivation much more clear and usecase very > sound. > > > This patch introduces the PROCESS_MRELEASE_REAP_KILL UAPI flag to support > > an integrated auto-kill mode. When specified, process_mrelease() directly > > injects a SIGKILL into the target task. > > > > To solve the race condition deterministically, we grab the mm reference > > via mmget() and set the MMF_UNSTABLE flag *before* sending the SIGKILL. > > Using mmget() instead of mmgrab() keeps mm_users > 0, preventing the > > victim from calling exit_mmap() in its own exit path. > > Why is this needed? Address space tear down is an operation that can run > from several execution contexts. Agreed. > > > This ensures that > > the memory is reclaimed synchronously and deterministically by the reaper > > in the context of process_mrelease(), avoiding delays caused by > > non-deterministic scheduling of the victim task. > > The memory is still reclaimed synchronously from the mrelease context. > This is really confusing. > > Please also explain why do you need to do all that ugly > task_will_free_mem hoops. Why cannot you simply kill the task if > task_will_free_mem fails (if PROCESS_MRELEASE_REAP_KILL is used). I wanted to handle shared address spaces. Even though we are okay with the target task not being in a SIGKILL state yet (since we are about to kill it), we must ensure that all *other* processes sharing the same mm are also dying. If we simply bypass the check and force a kill when there are living sharers, the memory will NOT be freed even after the target task dies because the other processes still pin the mm. So, to address this, I think we need to modify task_will_free_mem() slightly to ignore the exit state of the *target* task only, while still checking that all *other* sharing processes are dying: static bool task_will_free_mem(struct task_struct *task, bool ignore_exit) { ... /* ignore tarket task's signal state */ if (!__task_will_free_mem(task, ignore_exit)) return false; /* * but other processes sharing the mm with target must be exit * state */ for_each_process(p) { ... if (!__task_will_free_mem(p, false)) return false; } ... }