Re: [PATCH] x86/mm: Add an early PGT_validated exit in _get_page_type()

[Jan Beulich <jbeulich@xxxxxxxx>]

On 17.06.2022 12:47, Andrew Cooper wrote:
> This is a continuation of the cleanup and commenting in:
>   9186e96b199e ("x86/pv: Clean up _get_page_type()")
>   8cc5036bc385 ("x86/pv: Fix ABAC cmpxchg() race in _get_page_type()")
> 
> With the re-arranged and newly commented logic, it's far more clear that the
> second half of _get_page_type() only has work to do for page validation.

To be honest "far more clear" reads misleading to me: Part of the re-
arrangement was to move later the early setting of PGT_validated for
PGT_writable pages, without which the stated fact wasn't entirely true
prior to the re-arrangement. How about s/far more/now/ ?

> Introduce an early exit for PGT_validated.  This makes the fastpath marginally
> faster, and simplifies the subsequent logic as it no longer needs to exclude
> the fully validated case.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Preferably with the wording above adjusted:
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

> Not that it's relevant, but bloat-o-meter says:
> 
>   add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-300 (-300)
>   Function                                     old     new   delta
>   _get_page_type                              6618    6318    -300
> 
> which is more impressive than I was expecting.

And I have to admit I'm having trouble seeing why that would be.

Jan