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

[Julien Grall <julien.grall@xxxxxxx>]

On 03/05/2019 16:59, Andrii Anisov wrote:
> Hello Julien,
Hi,

> On 22.04.19 19:49, Julien Grall wrote:
> > The function create_xen_entries may be concurrently called. So we need
> > to protect with a spinlock to avoid corruption the page-tables.
> The question from me is why didn't we step into this issue before?
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).
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.
Cheers,

--
Julien Grall

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