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b=tVoTEZTIXsi3M+dPWmV/s1NPuMSPyF9gklAGL1CM0QIEjfXZnb7VTj0BRjE5DJKhm XWDicYFMzz/WEZmAG0Sd30py43F9pGiXqnWX7RLUKnToIJG8J8PgH2lLDQZBlJf/pW XWlwrzc6RifOjcE6bGVTTe+nUOhRWiPLQctj8fFVGW5E2JIWFb87hQ4SJSSq3V8PXV Lx6aKzhXLisOdHnU2OTLb/6lfWSRPEdequYf7ljUm65wxm805J3hlPDeKx8V6Oaj0U oLOpOiGDxW+xtm5IWcMDzNTqDfcajKauT58TYKi+guXNB7MsPxZKG6k6DcFjUIuldh E8tBkmuk0MevQ== Date: Mon, 20 Apr 2026 14:47:04 -0700 From: Minchan Kim To: Michal Hocko Cc: Christian Brauner , akpm@linux-foundation.org, david@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, surenb@google.com, timmurray@google.com Subject: Re: [RFC 3/3] mm: process_mrelease: introduce PROCESS_MRELEASE_REAP_KILL flag Message-ID: References: <20260413223948.556351-1-minchan@kernel.org> <20260413223948.556351-4-minchan@kernel.org> <20260416-planktont-abwinken-b9499483b939@brauner> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Stat-Signature: gfgb18ri1w1f3xis7jcnkptytgemn6fh X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 84E8FC0013 X-Rspam-User: X-HE-Tag: 1776721627-995784 X-HE-Meta: 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 HjH37dZQ Ws9wXcmEXnE/MHUzrsvWYdn/4zqJKZ557qGzbslaHG/z4/JDgdChPj/8pZNJoflJMQS/Cf9ICyH3d86QBrB/N+C+ZuKLcIbYEo2Hb3qBzVdaR9TkUAfjdPw5eGhOZ+mUdtOhrIi5UIV8ij39nn5MmAqvLdn87Q0dCTfsRyVHwYX+7BmRW/tuLXt03Rf7XV7NHy+aBhEn0c/CqxpTybqIbZFDHGOe+KTGwpKYo1VsCqXA5N7yEYG2o/3xuAA7U6SSWQClmzDJeN/8R6+Cx3V2IlU/r7sFU0g/Og03q7jP27mleFJn+zQVO+BLMzs9DvZiSpmSeGPEGdludl74= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Fri, Apr 17, 2026 at 09:04:31AM +0200, Michal Hocko wrote: > On Thu 16-04-26 23:30:09, Minchan Kim wrote: > > If I send the SIGKILL first to satisfy the process_mrelease() requirement, > > we immediately run into the scheduling race condition where the victim can > > enter the exit path before the reaper can set the flag. > > Why don't you just grab the mm before you send the signal and then continue > with reaping? You just want to avoid a race where the victim manages to > process fatal signal, start its exit path and mrelease path losing that > race so you rely on the exit path, right? The problem is that process_mrelease() operates on a task obtained from a pidfd. Once the victim process receives the SIGKILL and enters the exit path (exit_mm), the kernel sets task->mm to NULL. Even if we could somehow hold a reference to the mm_struct beforehand, process_mrelease() would still fail because mm_struct via task returns NULL after exit_mm() has been called. Therefore, we cannot simply "grab the mm" before sending the signal and expect process_mrelease() to work after the victim starts exiting.