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

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Message-ID: <YO55ZIrgkLXI4BbS@dhcp22.suse.cz>
Date:   Wed, 14 Jul 2021 07:43:00 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Evan Green <evgreen@...omium.org>,
        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 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