first calls mmap() with the arguments PROT_NONE and MAP_ANON to reserve a
single, contiguous range of virtual addresses for the entire shared library.
Later, rtld calls mmap() with the the shared library's file descriptor
and the argument MAP_FIXED to place the text and data sections within the
reserved range. The rationale for mapping shared libraries in this way is
explained in the commit message for Revision 190885. However, this approach
does have an unintended, negative consequence. Since the first call to
mmap() specifies MAP_ANON and not the shared library's file descriptor, the
kernel has no idea what alignment the vm object backing the file prefers.
As a result, the reserved range's alignment is unlikely to be the same as
the vm object's, and so mapping with superpages becomes impossible. To
address this problem, this revision adds the argument MAP_ALIGNED_SUPER to
the first call to mmap() if the text section is larger than the smallest
superpage size.
To determine if the text section is larger than the smallest superpage
size, rtld must always fetch the page size information. As a result, the
private code for fetching the base page size in rtld's builtin malloc is
redundant. Eliminate it. Requested by: kib
Tested by: zbb (on arm)
Reviewed by: kib (an earlier version)
Discussed with: jhb
ea8577c712