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Date:   Wed, 14 Jul 2021 07:43:00 +0200
From:   Michal Hocko <mhocko@suse.com>
To:     David Hildenbrand <david@redhat.com>
Cc:     Evan Green <evgreen@chromium.org>,
        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
Subject: Re: [PATCH v2] mm: Enable suspend-only swap spaces
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On Mon 12-07-21 09:41:26, David Hildenbrand wrote:
> 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.

Is there anything preventing such a HW event doing the equivalent of the
above?
-- 
Michal Hocko
SUSE Labs