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[37.188.179.51]) by smtp.gmail.com with ESMTPSA id v5sm192665wmh.12.2020.07.07.01.58.48 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 07 Jul 2020 01:58:48 -0700 (PDT) Date: Tue, 7 Jul 2020 10:58:47 +0200 From: Michal Hocko To: Pavel Machek Cc: Jann Horn , "Catangiu, Adrian Costin" , "linux-mm@kvack.org" , "linux-pm@vger.kernel.org" , "virtualization@lists.linux-foundation.org" , "linux-api@vger.kernel.org" , "akpm@linux-foundation.org" , "rjw@rjwysocki.net" , "len.brown@intel.com" , "fweimer@redhat.com" , "keescook@chromium.org" , "luto@amacapital.net" , "wad@chromium.org" , "mingo@kernel.org" , "bonzini@gnu.org" , "Graf (AWS), Alexander" , "MacCarthaigh, Colm" , "Singh, Balbir" , "Sandu, Andrei" , "Brooker, Marc" , "Weiss, Radu" , "Manwaring, Derek" Subject: Re: [RFC]: mm,power: introduce MADV_WIPEONSUSPEND Message-ID: <20200707085847.GA5913@dhcp22.suse.cz> References: <20200703113026.GT18446@dhcp22.suse.cz> <20200707073823.GA3820@dhcp22.suse.cz> <20200707080726.GA32357@amd> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200707080726.GA32357@amd> Sender: linux-pm-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-pm@vger.kernel.org On Tue 07-07-20 10:07:26, Pavel Machek wrote: > Hi! > > > > > > This patch adds logic to the kernel power code to zero out contents of > > > > > all MADV_WIPEONSUSPEND VMAs present in the system during its transition > > > > > to any suspend state equal or greater/deeper than Suspend-to-memory, > > > > > known as S3. > > > > > > > > How does the application learn that its memory got wiped? S2disk is an > > > > async operation and it can happen at any time during the task execution. > > > > So how does the application work to prevent from corrupted state - e.g. > > > > when suspended between two memory loads? > > > > > > You can do it seqlock-style, kind of - you reserve the first byte of > > > the page or so as a "is this page initialized" marker, and after every > > > read from the page, you do a compiler barrier and check whether that > > > byte has been cleared. > > > > This is certainly possible yet wery awkwar interface to use IMHO. > > MADV_EXTERNALY_VOLATILE would express the actual semantic much better. > > I might not still understand the expected usecase but if the target > > application has to be changed anyway then why not simply use a > > transparent and proper signaling mechanism like poll on a fd. That > > The goal is to have cryprographically-safe get_random_number() with 0 > syscalls. > > You'd need to do: > > if (!poll(did_i_migrate)) { > use_prng_seed(); > if (poll(did_i_migrate)) { > /* oops_they_migrated_me_in_middle_of_computation, > lets_redo_it() */ > goto retry: > } > } > > Which means two syscalls.. Is this a real problem though? Do we have any actual numbers? E.g. how often does the migration happen so that 2 syscalls would be visible in actual workloads? -- Michal Hocko SUSE Labs