linux-kernel - Re: [PATCH v2] mm: Enable suspend-only swap spaces

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Date:   Mon, 12 Jul 2021 09:41:26 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Michal Hocko <mhocko@...e.com>, Evan Green <evgreen@...omium.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Pavel Machek <pavel@....cz>, Alex Shi <alexs@...nel.org>,
        Alistair Popple <apopple@...dia.com>,
        Jens Axboe <axboe@...nel.dk>,
        Johannes Weiner <hannes@...xchg.org>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Minchan Kim <minchan@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-api@...r.kernel.org
Subject: Re: [PATCH v2] mm: Enable suspend-only swap spaces

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