Re: [Xen-devel] Converting heap page_infos to contiguous virtual

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 07/13/2016 04:02 PM, Andrew Cooper wrote:
> On 13/07/16 20:44, Boris Ostrovsky wrote:
>> I would like to clear a bunch of Xen heap pages at once (i.e. not
>> page-by-page).
>>
>> Greatly simplifying things, let's say I grab (in common/page_alloc.c)
>>     pg = page_list_remove_head(&heap(node, zone, order)
>>
>> and then
>>
>>     mfn_t mfn =
>> _mfn(page_to_mfn(pg));                                        
>>     char *va = mfn_to_virt(mfn_x(mfn));
>>     memset(va, 0, 4096 * (1 << order));
>>
>>
>> Would it be valid to this?
> In principle, yes.  The frame_table is in order.
>
> However, mfn_to_virt() will blow up for RAM above the 5TB boundary.  You
> need to map_domain_page() to get a mapping.

Right, but that would mean going page-by-page, which I want to avoid.

Now, DIRECTMAP_SIZE is ~128TB (if my math is correct) --- doesn't it
imply that it maps this big a range contiguously (modulo PDX hole)?

>
>>  Do I need to account for the PDX hole?
> Jan is probably the best person to ask about this, but I am failure sure
> there are lurking dragons here.
>
> PDX compression is used to reduce the size of the frametable when there
> are large unused ranges of mfns.  Without paying attention to the PDX
> shift, you don't know where the discontinuities lie.
>
> However, because the PDX shift is an aligned power of two, there are
> likely to be struct page_info*'s in the frame_table which don't point at
> real RAM, and won't have a virtual mapping even in the directmap.

So I would be OK with finding which mfn of my range points to beginning
of the hole and break the mfn range into two sections --- one below the
hole and one above. With hope that both ranges can be mapped
contiguously --- something that I don't know whether is true.

-boris




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