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Subject: Re: [PATCH v2] mm: Enable suspend-only swap spaces
To:     Michal Hocko <mhocko@suse.com>, Evan Green <evgreen@chromium.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Pavel Machek <pavel@ucw.cz>, Alex Shi <alexs@kernel.org>,
        Alistair Popple <apopple@nvidia.com>,
        Jens Axboe <axboe@kernel.dk>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        "Matthew Wilcox (Oracle)" <willy@infradead.org>,
        Miaohe Lin <linmiaohe@huawei.com>,
        Minchan Kim <minchan@kernel.org>,
        Vlastimil Babka <vbabka@suse.cz>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, linux-api@vger.kernel.org
References: <20210709105012.v2.1.I09866d90c6de14f21223a03e9e6a31f8a02ecbaf@changeid>
 <YOvpVRSMJe8NQuS2@dhcp22.suse.cz>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
Message-ID: <30dddfb1-388c-a593-0987-73e821216da9@redhat.com>
Date:   Mon, 12 Jul 2021 09:41:26 +0200
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On 12.07.21 09:03, Michal Hocko wrote:
> [Cc linux-api]
> 
> On Fri 09-07-21 10:50:48, Evan Green wrote:
>> Currently it's not possible to enable hibernation without also enabling
>> generic swap for a given swap area. These two use cases are not the
>> same. For example there may be users who want to enable hibernation,
>> but whose drives don't have the write endurance for generic swap
>> activities.
>>
>> Add a new SWAP_FLAG_NOSWAP 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.
> 
> Could you expand some more on why a strict exclusion is really
> necessary? I do understand that one might not want to have swap storage
> available all the time but considering that swapon is really a light
> operation so something like the following should be a reasonable
> workaround, no?
> 	swapon storage/file
> 	s2disk
> 	swapoff storage

I'm certainly not a hibernation expert, but I'd guess this can also be 
triggered by HW events, so from the kernel and not only from user space 
where your workaround would apply.

-- 
Thanks,

David / dhildenb