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Patch series "further cleanup of vma_merge()", v2.

Following on from Vlastimil Babka's patch series "cleanup vma_merge() and
improve mergeability tests" which was in turn based on Liam's prior
cleanups, this patch series introduces changes discussed in review of
Vlastimil's series and goes further in attempting to make the logic as
clear as possible.

Nearly all of this should have absolutely no functional impact, however it
does add a singular VM_WARN_ON() case.

With many thanks to Vernon for helping kick start the discussion around
simplification - abstract use of vma did indeed turn out not to be
necessary - and to Liam for his excellent suggestions which greatly
simplified things.


This patch (of 4):

Previously the ASCII diagram above vma_merge() and the accompanying
variable naming was rather confusing, however recent efforts by Liam
Howlett and Vlastimil Babka have significantly improved matters.

This patch goes a little further - replacing 'X' with 'N' which feels a
lot more natural and replacing what was 'N' with 'C' which stands for
'concurrent' VMA.

No word quite describes a VMA that has coincident start as the input span,
concurrent, abbreviated to 'curr' (and which can be thought of also as
'current') however fits intuitions well alongside prev and next.

This has no functional impact.

Link: https://lkml.kernel.org/r/cover.1679431180.git.lstoakes@gmail.com
Link: https://lkml.kernel.org/r/6001e08fa7e119470cbb1d2b6275ad8d742ff9a7.1679431180.git.lstoakes@gmail.com

Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Vernon Yang <vernon2gm@gmail.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>