Re: [Xen-devel] [PATCH 16/20] xen/arm: mm: Protect Xen page-table update with a spinlock

[Andrii Anisov <andrii.anisov@xxxxxxxxx>]

Hello Julien,

On 03.05.19 20:19, Julien Grall wrote:
> TLDR; because xen page-tables are not that often modified after boot. Yet it is 
> still possible to race.
>
> At the moment, create_xen_entries() can only modify the VA range 0 - 2GB. In 
> that range, we can modify at runtime the VMAP area. One potential issue is
> a vmap issued at the same time.
>
> While the range allocation is protected by a lock (see vm_alloc), the mapping 
> is not. So it would be possible to end up modifying the page-table at the same. 
> That could blow up if for instance, the second-level entry is invalid as we 
> would need to allocate memory (only one can win that race).
I understand the potential race, but still wondering why didn't we see those 
issues. Maybe we are too lucky.

> In general, it is a saner approach to try to serialize the modifications in the 
> page-tables. So you can safely read an entry, check it and then update it.
Yet, I think we would stick at these locks for now.

--
Sincerely,
Andrii Anisov.

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