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

[David Woodhouse <dwmw2@infradead.org>]

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On Wed, 2020-02-05 at 10:22 +0000, Julien Grall wrote:
> Hi,
> 
> On 05/02/2020 09:50, 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?
> > 
> > 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) )
> 
> Note that there is two versions of free_xenheap_pages(). This one only 
> cover the case where the domheap and xenheap are the same.
> 
> But you can't use the same trick when xenheap is separated (like on 
> Arm32) because PGC_xen_heap is not set. So you would be calling 
> init_heap_pages() everytime.

Right. We'd want to set PGC_xen_heap there too on the corresponding
pages.

> However, I don't like the idea of relying on count_info == 0. Indeed, 
> there are valid case where count_info == 0 because it means the page is 
> inuse (PCC_state_inuse).

For xenheap pages, not just domheap pages?

> It might be best if we introduce a new page state that would be the 
> default value set in the frametable.

... and which is easily set with memset() so it's fast, which I think
you considered since you were suggesting all 0xFF, but which we haven't
explciitly stated out loud.

The other thing to note is that it's easy to make a default state for
pages which *weren't* given out by the boot allocator, but we don't
have a simple way to do anything to the pages which *were* given out by
the boot allocator, since we don't track those and — fairly much by
definition — we don't have the frametable yet when we start the boot
allocator.



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