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

Pavel Machek
2020-07-07T08:07:33Z

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Hi!

> > > > 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

The goal is to have cryprographically-safe get_random_number() with 0
syscalls.

You'd need to do:

if (!poll(did_i_migrate)) {
use_prng_seed();
if (poll(did_i_migrate)) {
/* oops_they_migrated_me_in_middle_of_computation,
lets_redo_it() */
goto retry:
}
}

Which means two syscalls..

Best regards,

Pavel
--
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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