On Fri, Jul 9, 2021 at 3:20 PM Andrew Morton [off-list ref] wrote:

On Fri,  9 Jul 2021 10:50:48 -0700 Evan Green [off-list ref] wrote:

quoted

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?

There's no other patch, in the kernel at least. SWAP_FLAG_* is exposed
to usermode, which would set it when calling swapon(2). Once this
patch is accepted, I'll have to add the option into util-linux [1], so
that I can use it in my init scripts.

Said a different way, this patch isn't about altering how hibernate
behaves, but about giving usermode the freedom to set up hibernate and
swap independently.

[1] https://github.com/karelzak/util-linux/blob/b4533177aeac287e0b0563cd1b9ee61bce29ee88/sys-utils/swapon.c#L710

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?

The above paragraph maybe cleared this up. The hibernate code will
still happily do I/O to any region handed to it by the swap code. That
could be a region already peppered with active swap (the normal case
today), or a NOSWAP region which swap otherwise stays out of (but
still manages). If we did swapoff and swapon with a different setting
for NOSWAP, that should all work fine, since hibernate leaves it to
the swapfile code to be in charge of sector allocs/frees.

There shouldn't be any backward compatibility issues because
SWAP_FLAGS_VALID enforced that usermode had been keeping the
unallocated bits as 0.

-Evan