Re: [Xen-devel] [XEN PATCH v2 1/2] Check zone before merging adjacent blocks in heap

[David Woodhouse <dwmw2@xxxxxxxxxxxxx>]

On Tue, 2020-02-04 at 15:37 +0000, George Dunlap wrote:
> At very least it's more robust this way; the algorithm is also less
> "magic".  We just had a long discussion this morning trying to re-create
> the logic for why "Remove MFN 0" was sufficient to prevent this issue,
> and even then David wasn't sure it was correct at first.

Right. So the real reason I'm staring hard at this is because I can't
convince myself there aren't places where memory allocated by the boot
allocator is later freed with free_xenheap_pages().

We have a few pieces of code which decide whether to use the boot
allocator vs. heap based on system_state >= SYS_STATE_boot, and *if*
the rule is "thou shalt not allocate with boot allocator and free
later" then it's *extremely* fragile and probably being violated —
especially because it actually *works* most of the time, except in some
esoteric corner cases like MFN#0, boot allocations which cross
zones/regions, etc.

So because we want to make that *more* likely by allowing vmap() to
happen earlier, I'd like to clean things up by addressing those corner
cases and making it unconditionally OK to free boot-allocated pages
into the heap.

I think might be as simple as checking for (first_pg)->count_info == 0
in free_xenheap_pages(). That's quick enough, and if the count_info is
zero then I think it does indicate a boot-allocated page, because pages
from alloc_xenheap_pages() would have PGC_xen_heap set?

It would suffice just to pass such pages to init_heap_pages() instead
of directly to free_heap_pages(), I think. Julien?

The straw man version of that looks a bit like this...

--- a/xen/common/page_alloc.c
+++ b/xen/common/page_alloc.c
@@ -2304,6 +2304,12 @@ void free_xenheap_pages(void *v, unsigned int order)
 
     pg = virt_to_page(v);
 
+    /* Pages from the boot allocator need to pass through init_heap_pages() */
+    if ( unlikely(!pg->count_info) )
+    {
+        init_heap_pages(pg, 1 << order);
+        return;
+    }
     for ( i = 0; i < (1u << order); i++ )
         pg[i].count_info &= ~PGC_xen_heap;

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