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

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Date:   Fri, 9 Jul 2021 15:20:24 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Evan Green <evgreen@...omium.org>
Cc:     David Hildenbrand <david@...hat.com>, 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>,
        Michal Hocko <mhocko@...e.com>,
        Minchan Kim <minchan@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH v2] mm: Enable suspend-only swap spaces

On Fri,  9 Jul 2021 10:50:48 -0700 Evan Green <evgreen@...omium.org> 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.
> 
> Swap regions with SWAP_FLAG_NOSWAP set will not appear in /proc/meminfo
> under SwapTotal and SwapFree, since they are not usable as general swap.
> 

This patch doesn't appear to set SWAP_FLAG_NOSWAP anywhere.  Perhaps
there's another patch somewhere which changes the hibernation code?  If
so, can we please have both patches in a series?

Once we have a description of how this thing gets set, please let's
discuss what happens if someone tries to enable generic swap onto that
device after hibernation has set SWAP_FLAG_NOSWAP (I'm basically
guessing now).  Will it work?  Is there a backward-compatibility issue
here?