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[79.242.60.54]) by smtp.gmail.com with ESMTPSA id k9sm3199009wrc.6.2021.07.27.05.21.28 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 27 Jul 2021 05:21:28 -0700 (PDT) Subject: Re: [PATCH v4] mm: Enable suspend-only swap spaces From: David Hildenbrand To: Evan Green , Andrew Morton Cc: Michal Hocko , Pavel Machek , linux-api@vger.kernel.org, Alex Shi , Alistair Popple , Johannes Weiner , Joonsoo Kim , "Matthew Wilcox (Oracle)" , Miaohe Lin , Minchan Kim , Suren Baghdasaryan , Vlastimil Babka , linux-kernel@vger.kernel.org, linux-mm@kvack.org References: <20210726171106.v4.1.I09866d90c6de14f21223a03e9e6a31f8a02ecbaf@changeid> Organization: Red Hat Message-ID: <6ff28cfe-1107-347b-0327-ad36e256141b@redhat.com> Date: Tue, 27 Jul 2021 14:21:27 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 27.07.21 11:48, David Hildenbrand wrote: > On 27.07.21 02:12, Evan Green wrote: >> Add a new SWAP_FLAG_HIBERNATE_ONLY that adds a swap region but refuses >> to allow generic swapping to it. This region can still be wired up for >> use in suspend-to-disk activities, but will never have regular pages >> swapped to it. This flag will be passed in by utilities like swapon(8), >> usage would probably look something like: swapon -o hibernate /dev/sda2. >> >> Currently it's not possible to enable hibernation without also enabling >> generic swap for a given area. One semi-workaround for this is to delay >> the call to swapon() until just before attempting to hibernate, and then >> call swapoff() just after hibernate completes. This is somewhat kludgy, >> and also doesn't really work to keep swap out of the hibernate region. >> When hibernate begins, it starts by allocating a large chunk of memory >> for itself. This often ends up forcing a lot of data out into swap. By >> this time the hibernate region is eligible for generic swap, so swap >> ends up leaking into the hibernate region even with the workaround. >> >> There are a few reasons why usermode might want to be able to >> exclusively steer swap and hibernate. One reason relates to SSD wearing. >> Hibernate's endurance and speed requirements are different from swap. >> It may for instance be advantageous to keep hibernate in primary >> storage, but put swap in an SLC namespace. These namespaces are faster >> and have better endurance, but cost 3-4x in terms of capacity. >> Exclusively steering hibernate and swap enables system designers to >> accurately partition their storage without either wearing out their >> primary storage, or overprovisioning their fast swap area. >> >> Another reason to allow exclusive steering has to do with security. >> The requirements for designing systems with resilience against >> offline attacks are different between swap and hibernate. Swap >> effectively requires a dictionary of hashes, as pages can be added and >> removed arbitrarily, whereas hibernate only needs a single hash for the >> entire image. If you've set up block-level integrity for swap and >> image-level integrity for hibernate, then allowing swap blocks to >> possibly leak out to the hibernate region is problematic, since it >> creates swap pages not protected by any integrity. >> >> Swap regions with SWAP_FLAG_HIBERNATE_ONLY set will not appear in >> /proc/meminfo under SwapTotal and SwapFree, since they are not usable as >> general swap. These regions do still appear in /proc/swaps. > > Right, and they also don't account towards the memory overcommit > calculations. > > Thanks for extending the patch description! > > [...] > >> + if (swap_flags & SWAP_FLAG_HIBERNATE_ONLY) { >> + if (IS_ENABLED(CONFIG_HIBERNATION)) { >> + if (swap_flags & ~SWAP_HIBERNATE_ONLY_VALID_FLAGS) >> + return -EINVAL; >> + >> + } else { >> + return -EINVAL; >> + } >> + } > > We could do short > > if ((swap_flags & SWAP_FLAG_HIBERNATE_ONLY) && > (!IS_ENABLED(CONFIG_HIBERNATION) || > (swap_flags & ~SWAP_HIBERNATE_ONLY_VALID_FLAGS))) > return -EINVAL; > > or > > if (swap_flags & SWAP_FLAG_HIBERNATE_ONLY)) > if (!IS_ENABLED(CONFIG_HIBERNATION) || > (swap_flags & ~SWAP_HIBERNATE_ONLY_VALID_FLAGS)) > return -EINVAL; > >> + >> if (!capable(CAP_SYS_ADMIN)) >> return -EPERM; >> >> @@ -3335,16 +3366,20 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags) >> if (swap_flags & SWAP_FLAG_PREFER) >> prio = >> (swap_flags & SWAP_FLAG_PRIO_MASK) >> SWAP_FLAG_PRIO_SHIFT; >> + >> + if (swap_flags & SWAP_FLAG_HIBERNATE_ONLY) >> + p->flags |= SWP_HIBERNATE_ONLY; >> enable_swap_info(p, prio, swap_map, cluster_info, frontswap_map); >> >> - pr_info("Adding %uk swap on %s. Priority:%d extents:%d across:%lluk %s%s%s%s%s\n", >> + pr_info("Adding %uk swap on %s. Priority:%d extents:%d across:%lluk %s%s%s%s%s%s\n", >> p->pages<<(PAGE_SHIFT-10), name->name, p->prio, >> nr_extents, (unsigned long long)span<<(PAGE_SHIFT-10), >> (p->flags & SWP_SOLIDSTATE) ? "SS" : "", >> (p->flags & SWP_DISCARDABLE) ? "D" : "", >> (p->flags & SWP_AREA_DISCARD) ? "s" : "", >> (p->flags & SWP_PAGE_DISCARD) ? "c" : "", >> - (frontswap_map) ? "FS" : ""); >> + (frontswap_map) ? "FS" : "", >> + (p->flags & SWP_HIBERNATE_ONLY) ? "H" : ""); >> >> mutex_unlock(&swapon_mutex); >> atomic_inc(&proc_poll_event); >> > > Looks like the cleanest alternative to me, as long as we don't want to > invent new interfaces. > > Acked-by: David Hildenbrand > Pavel just mentioned uswsusp, and I wonder if it would be a possible alternative to this patch. -- Thanks, David / dhildenb