Re: [RFC]: mm,power: introduce MADV_WIPEONSUSPEND

Michal Hocko
2020-07-07T07:38:29Z

On Fri 03-07-20 15:29:22, Jann Horn wrote:
> On Fri, Jul 3, 2020 at 1:30 PM Michal Hocko <mhocko@kernel.org> wrote:
> > On Fri 03-07-20 10:34:09, Catangiu, Adrian Costin wrote:
> > > This patch adds logic to the kernel power code to zero out contents of
> > > all MADV_WIPEONSUSPEND VMAs present in the system during its transition
> > > to any suspend state equal or greater/deeper than Suspend-to-memory,
> > > known as S3.
> >
> > How does the application learn that its memory got wiped? S2disk is an
> > async operation and it can happen at any time during the task execution.
> > So how does the application work to prevent from corrupted state - e.g.
> > when suspended between two memory loads?
>
> You can do it seqlock-style, kind of - you reserve the first byte of
> the page or so as a "is this page initialized" marker, and after every
> read from the page, you do a compiler barrier and check whether that
> byte has been cleared.

This is certainly possible yet wery awkwar interface to use IMHO.
MADV_EXTERNALY_VOLATILE would express the actual semantic much better.
I might not still understand the expected usecase but if the target
application has to be changed anyway then why not simply use a
transparent and proper signaling mechanism like poll on a fd. That would
be certainly a more natural and less error prone programming interface.

--
Michal Hocko
SUSE Labs