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

[Juergen Gross <jgross@xxxxxxxx>]

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
> else.
>
> 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
> concerning.
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.


Juergen

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