RE: [Xen-devel] slow xp hibernation revisited

["James Harper" <james.harper@xxxxxxxxxxxxxxxx>]

> AIUI the logic in the mapcache is something like:
>  - Each bucket contains a number of 'locked' mappings (which aren't
used
>    for this kind of copy).
>  - At the bottom of each bucket is a possible 'unlocked' mapping.
>  - If the unlocked mapping matches the address you want, reuse it
>  - Else discard it and replace it with a new unlocked mapping to your
>    target area.
> 
> But something is going on and the "else" clause is happening every
> time.
> 

It's the !test_bit(address_offset>>XC_PAGE_SHIFT, entry->valid_mapping)
that is causing the if expression to be true. From what I can see so
far, the bit representing the pfn in entry->valid_mapping is 0 because
err[] returned for that pfn was -EINVAL.

Maybe the test is superfluous? Is there a need to do the remap if all
the other variables in the expression are satisfied? If the remap was
already done and the page could not be mapped last time, what reasons
are there why it would succeed this time?

James

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