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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 05.02.2020 10:50, David Woodhouse wrote:
> 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?

They would, but that leaves {alloc,free}_domheap_pages() out of
the picture. I.e. ...

> 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) )

... while I think this check may be fine here, no similar one
can be used in free_domheap_pages(), yet pages getting handed
there isn't less likely than ones getting handed to
free_xenheap_pages() (if we already fear mismatch).

Jan

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