Re: [Xen-ia64-devel] Modify to introduce delayed p2m table destruction

[Isaku Yamahata <yamahata@xxxxxxxxxxxxx>]

On Mon, Nov 06, 2006 at 03:15:22PM +0900, Doi.Tsunehisa@xxxxxxxxxxxxxx wrote:
> >>> However during shadow_teardown_xxx() in your patch
> >>> other domain might access the p2m table and struct page_info.
> >>> Page reference convension must be kept right during them.
> >> 
> >>   Yes, it might access them. In past, I thought so, but after
> >> discussion about delayed p2m table destruction of shadow2, I've be
> >> satisfied that get_page avoids memory corruption finally.
> > 
> > You might understand x86 shadow code.
> > However you must understand IA64 code too.
> > It would be effective to understand IA64 code by analogy of
> > x86 shadow code, But they're different.
> 
>   Hmm, I don't understand the difference.
> 
>   Can you give me suggestion about the difference ?

The Xen/IA64 p2m table is lockless while Xen/x86 shadow p2m table 
is protected by shadow_lock()/shadow_unlock().
This is a burden to the Xen/IA64 p2m maintenance.
So we must be very carefull when modifying it. 
Especially we must be aware of memory ordering.
This is the reason why volatile is sprinckled.

In Xen/IA64 p2m case
page reference count must be increased before you add the new entry,
page reference count must be decreased after removing the entry.
The only exception is relinquish_pte() because it assumes that 
the p2m itself is freed. (But this assumption is wrong.)
However Xen/x86 shadow p2m doesn't care abount page reference count.

The blktap patches which I sent out last night impose a one more new rule
which is related to PGC_allocated flag.
The patch introduces _PAGE_PGC_ALLOCATED.
When the p2m entry is removed and _PAGE_PGC_ALLOCATED bit is set,
something like
if (pte_pgc_allocated(old_pte)) {
    if (test_and_clear(_PGC_allocated, &page->count_info))
        put_page(page)
}
must be done. domain_put_page() takes care of it.

Thanks.
-- 
yamahata

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