Re: [PATCH 1/2] x86: Perform mem_sharing teardown before paging teardown

[Tamas K Lengyel <tamas@xxxxxxxxxxxxx>]

On Thu, Feb 16, 2023 at 3:31 AM Jan Beulich <jbeulich@xxxxxxxx> wrote:
>
> On 15.02.2023 17:29, Tamas K Lengyel wrote:
> > On Wed, Feb 15, 2023 at 5:27 AM Jan Beulich <jbeulich@xxxxxxxx> wrote:
> >> Did you consider the alternative of adjusting the ASSERT() in question
> >> instead? It could reasonably become
> >>
> >> #ifdef CONFIG_MEM_SHARING
> >>     ASSERT(!p2m_is_hostp2m(p2m) || !remove_root ||
> > !atomic_read(&d->shr_pages));
> >> #endif
> >>
> >> now, I think. That would be less intrusive a change (helpful for
> >> backporting), but there may be other (so far unnamed) benefits of pulling
> >> ahead the shared memory teardown.
> >
> > I have a hard time understanding this proposed ASSERT.
>
> It accounts for the various ways p2m_teardown() can (now) be called,
> limiting the actual check for no remaining shared pages to the last
> of all these invocations (on the host p2m with remove_root=true).
>
> Maybe
>
>     /* Limit the check to the final invocation. */
>     if ( p2m_is_hostp2m(p2m) && remove_root )
>         ASSERT(!atomic_read(&d->shr_pages));
>
> would make this easier to follow? Another option might be to move
> the assertion to paging_final_teardown(), ahead of that specific call
> to p2m_teardown().

AFAICT d->shr_pages would still be more then 0 when this is called before sharing is torn down so the rearrangement is necessary even if we do this assert only on the final invocation. I did a printk in place of this assert without the rearrangement and afaict it was always != 0.

Tamas