Re: [PATCH] mm/mprotect: Fix maple tree start address in do_mprotect_pkey()

[Liam Howlett]

* Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> [220825 21:06]:
> On Thu, 25 Aug 2022 20:30:24 +0000 Liam Howlett <liam.howlett@xxxxxxxxxx> wrote:
> 
> > Use the untagged_addr() instead of the address passed into the function.
> 
> What are the runtime effects of this fix?

Prior to my change to use the maple tree, the start address was changed
before calling find_vma() with the untagged_addr() version of start.  My
first change recorded the tagged address and searched on the incorrect
start location - which would have found the incorrect VMA.  This fix
will use the untagged_addr() as the start of the search as it was
before I changed the code at all.

Any penalty of calling untagged_addr() occurred regardless of the
version that was used.  The search of the maple tree would have also
occurred in both versions - just at the wrong location before this fix.
I expect that the execution time would be equal as the search on the
tagged address would have either returned a VMA at start, or the VMA in
the next slot in the maple tree node - probably immeasurably slower
since the data is very likely already in the CPU cache, but I don't have
hard data to say either way.  I can look into a benchmark to measure the
difference between both working versions, but I don't have an arm64
native target so it will be emulated.