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

[Tamas K Lengyel <tamas@xxxxxxxxxxxxx>]

On Thu, Feb 16, 2023 at 9:27 AM Jan Beulich <jbeulich@xxxxxxxx> wrote:
>
> On 16.02.2023 13:22, Tamas K Lengyel wrote:
> > 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.
>
> Was your printk() in an if() as above? paging_final_teardown() runs really
> late during cleanup (when we're about to free struct domain), so I would
> be somewhat concerned if by that time the count was still non-zero.

Just replaced the assert with a printk. Without calling relinquish shared pages I don't find it odd that the count is non-zero, it only gets decremented when you do call relinquish. Once the order is corrected it is zero.

Tamas