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Re: [PATCH v2 03/13] libxenguest: deal with log-dirty op stats overflow

On 19.08.21 13:06, Jan Beulich wrote:
On 19.08.2021 12:20, Juergen Gross wrote:
On 05.07.21 17:13, Jan Beulich wrote:
In send_memory_live() the precise value the dirty_count struct field
gets initialized to doesn't matter much (apart from the triggering of
the log message in send_dirty_pages(), see below), but it is important
that it not be zero on the first iteration (or else send_dirty_pages()
won't get called at all). Saturate the initializer value at the maximum
value the field can hold.

While there also initialize struct precopy_stats' respective field to a
more sane value: We don't really know how many dirty pages there are at
that point.

In suspend_and_send_dirty() and verify_frames() the local variables
don't need initializing at all, as they're only an output from the
hypercall which gets invoked first thing.

In send_checkpoint_dirty_pfn_list() the local variable can be dropped
altogether: It's optional to xc_logdirty_control() and not used anywhere

Note that in case the clipping actually takes effect, the "Bitmap
contained more entries than expected..." log message will trigger. This
being just an informational message, I don't think this is overly

Is there any real reason why the width of the stats fields can't be
expanded to avoid clipping? This could avoid the need to set the
initial value to -1, which seems one of the more controversial changes.

While not impossible, it comes with a price tag, as we'd either need
to decouple xc_shadow_op_stats_t from struct xen_domctl_shadow_op_stats
or alter the underlying domctl. Neither of which looked either

I was thinking about the domctl.

Apart of struct xen_sysctl_page_offline_op this seems to be the only
left domctl/sysctl structure limiting guest or host size to values
being relevant. Changing those would be a sensible thing to do IMO.

appealing or necessary to me; instead I'm still struggling with
Andrew's comments, yet I didn't receive any clarification of further
explanation. Plus I continue to think that statistics output like this
shouldn't be assumed to be precise anyway, and for practical purposes
I don't think it really matters how large the counts actually are once
they've moved into the billions.

In case the underlying problem can easily be avoided be changing the
type of the field I don't see why this shouldn't be done. Especially as
in this case a misleading message will just be avoided.


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