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Re: [PATCH v4 6/6] xen/ppc: mm-radix: Allocate all paging structures at runtime
- To: Shawn Anastasio <sanastasio@xxxxxxxxxxxxxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Fri, 27 Sep 2024 08:22:25 +0200
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- Cc: tpearson@xxxxxxxxxxxxxxxxxxxxx, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Fri, 27 Sep 2024 06:22:30 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 27.09.2024 00:14, Shawn Anastasio wrote:
> On 4/24/24 9:46 AM, Jan Beulich wrote:
>> On 12.04.2024 05:55, Shawn Anastasio wrote:
>>> + /*
>>> + * Identity map all pages we've allocated for paging structures. This
>>> act
>>> + * itself will allocate more pages, so continue until we've mapped from
>>> + * `max_alloc_mfn` down to `min_alloc_mfn`. This assumes that the heap
>>> grows
>>> + * downwards, which matches the behavior of alloc_boot_pages.
>>
>> ... as long as you only ever allocate individual pages (which looks
>> to be the case for such incremental allocations, but that imo doesn't
>> go without saying).
>>
>
> I'm not sure I'm following you. `initial_page_alloc` as well as its
> underlying `alloc_boot_pages` can only ever allocate individual pages
> (or contiguous ranges of pages).
It's this "or contiguous ranges of pages" which constitutes a problem.
Once that happens, a portion of un-allocated pages may remain above
that block. A subsequent single-page allocation may then use one of
those pages, resulting in strict ordering to no longer hold. Or the
space may remain un-allocated, and hence being mapped for no reason.
Jan
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