Re: [RFC]: userspace memory reaping

On Thu, Nov 05, 2020 at 08:50:58AM -0800, Suren Baghdasaryan wrote:

On Thu, Nov 5, 2020 at 4:20 AM Michal Hocko [off-list ref] wrote:

quoted

On Wed 04-11-20 12:40:51, Minchan Kim wrote:

quoted

On Wed, Nov 04, 2020 at 07:58:44AM +0100, Michal Hocko wrote:

quoted

On Tue 03-11-20 13:32:28, Minchan Kim wrote:

quoted

On Tue, Nov 03, 2020 at 10:35:50AM +0100, Michal Hocko wrote:

quoted

On Mon 02-11-20 12:29:24, Suren Baghdasaryan wrote:
[...]

quoted

To follow up on this. Should I post an RFC implementing SIGKILL_SYNC
which in addition to sending a kill signal would also reap the
victim's mm in the context of the caller? Maybe having some code will
get the discussion moving forward?

Yeah, having a code, even preliminary, might help here. This definitely
needs a good to go from process management people as that proper is land
full of surprises...

Just to remind a idea I suggested to reuse existing concept

    fd = pidfd_open(victim process)
    fdatasync(fd);
    close(fd);

I must have missed this proposal. Anyway, are you suggesting fdatasync
to act as a destructive operation?

write(fd) && fdatasync(fd) are already destructive operation if the file
is shared.

I am likely missing something because fdatasync will not destroy any
underlying data. It will sync

quoted

You don't need to reaping as destruptive operation. Rather than, just
commit on the asynchrnous status "write file into page cache and commit
with fsync" and "killing process and commit with fsync".

I am sorry but I do not follow. The result of the memory reaping is a
data loss. Any private mapping will simply lose it's content. The caller
will get EFAULT when trying to access it but there is no way to
reconstruct the data. This is everything but not resembling what I see
f{data}sync is used for.

I think Minchan considers f{data}sync as a "commit" operation. So
write+f{data}sync would mean we write and commit written data,
kill+f{data}sync would mean we kill and commit that kill (reclaim the
resources).

If people doesn't like f{data}sync, ftruncate? My point is let's reuse
exising API since we have pidfd.

What I don't like about SIGKILL_SYNC is that it might introduce several
SIGXXX_SYNC later.