On Fri, Jul 9, 2021 at 3:20 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote: > > On Fri, 9 Jul 2021 10:50:48 -0700 Evan Green <evgreen@xxxxxxxxxxxx> 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? 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
