In subsequent commits we are going to determine KSM eligibility prior to a
VMA being constructed, at which point we will of course not yet have access
to a VMA pointer.
It is trivial to boil down the check logic to be parameterised on
mm_struct, file and VMA flags, so do so.
As a part of this change, additionally expose and use file_is_dax() to
determine whether a file is being mapped under a DAX inode.
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
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